Recoating the hull gell coat...

[mklein@iowatelecom.net]

Hi, I am refinishing an old mac 25. It has many scrapes in the gell coat. I was wondering after filling all of the old scrapes and sanding smooth, can I mix up and spray the entire surface with a thin coat of new gell coat in order to even out the color differences? Or should I just paint it? The painted sailboats that I have always get rub spots on the bow at the trailer vee stop... I know my nomenclature is not right, but you should get the idea. All comments are appreciated. Thanks Mark

 

[tommays]

Could it be done YES


But it is pretty complex to get right and does not really lend it self to the DIY

 

[mklein@iowatelecom.net]

Why not? It seems the gell coat could be thinnned with MEK and sprayed like a paint. The difficult thing would be cleaning the spray gun...

 

[dscribner]

Gelcoat needs to be THICK. Then there's the hardener and mold release issue. I really don't know what I'm talking about but I'm currently using Don Casey's book on sailboat refinishing to prepare and paint mine. Before you follow that path, do you're home work. I think the stuff is very self-leveling. Could it be roll-and-tipped?

 

[RichH]

Gel is easy to spray. Gel coat is easy to 'color-match' to the existing color.
All it takes is time & money.

You'll need a gel-coat compatible spray gun.... or Preval aerosol sprayers for (spot repairs, etc.), lots of elbow grease to flat-sand the gel back to 'flat', then extensive power buffing, etc.

Painting a hull relegates one to 'always' painting the hull on a continued 'maintenance' basis as 'most' topside paints are incompatible with long term water immersion !!!!!!!

A good source for Gel coat supplies Ive found is: gelcoatproducts.com/ in Seattle --- good prices, great turnaround on orders, precise color-matching .... but not an informative website for the DIYer.

 

[kenn]

Most gelcoat is designed to be cured in a mold, without any air contact, so you can't just buy any old gelcoat and paint it on.

I'm in about the same situation as you, Mark. I stripped all the ancient anti-fouling off of our little boat a couple of years ago, and got it down to the gel-coat. There was some gelcoat damage; I filled nicks gouges and scratches with a special air-cure gelcoat - Evercoat one-step finish gel coat (PDF file). This stuff, when mixed up, is like thick paint. That's good for scratches, but requires multiple coats to fill anything deeper.

For below the waterline, I am going to finish off with a good epoxy bottom paint, then a hard antifouling like VC-17. For above the waterline, even though the boat is 30 years old, I got pretty good results by very light sanding and buffing the gelcoat with fiberglass rubbing compound, and polishing. (look for info here on Maine Sail's excellent polishing instructions)

If it were any worse, I would aim to paint the hull, rather than attempting to gelcoat the whole thing. I've seen some great, almost factory finishes from using the better brands of boat paint with just careful rolling and tipping with a brush.

 

[monkeybars]

gel coat is just pigmented polyester resin with some additional additives.it is either waxed or unwaxed.all poly resins must be waxed or sprayed with mold release on final coat for a hard finish cure,you leave it unwaxed when additional coats are to be applied on top of eachother.don't be afraid to do your boat it is not magic,do small sections at a time i.e your bow area where you might want 3-4 coats,spray or foam brush will work just need to DA sand the brush job nore.

 

[RichH]

If the hull has been 'flat-sanded' (1000-2000 grit wet & dry paper) back to 'good' color, you can most times use 'color matched' (extra charge) gelcoat to make the repairs of gouges, etc. For color matching you cut out a 1" diameter 'sample' and send that to be color analysed on a 'color spectrometer'. You buy 'paste' for the 'deep' repairs and 'thinned out' gelcoat already mixed for spraying .... or can add something called 'patch aid' to mix up your own thinned out gel for spraying.

You first use 'paste' gelcoat to make the 'fill', then spray over and 'blend' the surface (and a wee bit of surrounding area) with a 'feathered-out' spray of gelcoat; can be done easily with 'small repairs'.
Best is not to use 'air inhibited' gelcoat or gelcoat WITH wax added ..... as youre going to flat-sand and 'power buff' it all anyway. If you are going to apply MANY coats and over large areas, then you can use 'air inhibited' (wax added) gel so that you dont have to sand between coats.

 

[CharlieCobra]

As far as Gelcoat curing when sprayed issues go, there's an additive for the Gelcoat that forms a waxing film over it when sprayed to allow quick curing. Gelcoat is easy with a decent compressor and gun. Be sure to sand the old Gelcoat to provide mechanical teeth to the new.

Also, de-wax the boat prior to sanding. Waxes ground into the old Gelcoat will make life miserable when you apply the new....

 

[lee5267]

Thin coat of gel coat??

I am no expert but I have painted vehicles, airplanes, and motorcycles with good results and I believe my information applies here. I would be quite hesitant to spray a thin layer of gel coat due to the likelyhood of having many many spider cracks all in the thin layer in a very short period of time. Gel coat finishes to a mostly non flexible finish as can be seen by the small hairline cracks we get in our boats over time where the fiberglass flexes more than the gel coat is capable of handling, causing cracks in areas that experience more flex. When you paint areas that will be flexed frequently or that have the ability to flex (car bumpers for example) you add a flexing agent to the paint so that when the bumper flexes the paint does not crack but instead flexes with the material it is bound to. I dont know of a flexing agent to go with gel coat. I personally believe that the reason we dont see lots and lots of spider cracks all over the boat is because generally there is a pretty thick layer of gel coat that goes down first during the production of the boat which helps reduce flex in general, and being a thick layer helps the initial gel coat resist cracking overall since boats dont usually experience tons of flex anyway. However if you were to just put a small thin layer of gel coat over the thick layer you risk it cracking very easily unless it completely and perfectly bonds/molds itself into the existing gel coat at the molecular level. I dont believe this perfect bond would happen since the old gel coat is completely cured and the new layer would simply adhere to the small scratches you would make when preparing the surface with 400grit wet sanding. The only way to get the perfect bond would be when you spray another layer of gel coat on a tacky layer that hasnt been completely cured.

Again this is just my two cents based on my previous knowledge of how most paint substances work, and my experience of putting non flexing paint on a surface that may experience flex (very bad after all the hard work). If you want a thin layer then I recommend paint that needs to be re-done every few yrs, or put on a heavier layer of gel coat that will be less likely to crack.

 

[monkeybars]

Sorry lee but paint and gelcoat don't behave the same way,in the xamples you reference.

 

[Maine Sail]

I personally believe that the reason we dont see lots and lots of spider cracks all over the boat is because generally there is a pretty thick layer of gel coat that goes down first during the production of the boat which helps reduce flex in general, and being a thick layer helps the initial gel coat resist cracking overall since boats dont usually experience tons of flex anyway.

Gelcoat will do little to nothing to prevent hull flexing. The thicker the gelcoat the more prone it is to crazing and cracking. Cape Dory laid on their gel perhaps thicker than anyone and their boats experience tons of crazing because of this.

However if you were to just put a small thin layer of gel coat over the thick layer you risk it cracking very easily unless it completely and perfectly bonds/molds itself into the existing gel coat at the molecular level. I dont believe this perfect bond would happen since the old gel coat is completely cured and the new layer would simply adhere to the small scratches you would make when preparing the surface with 400grit wet sanding. The only way to get the perfect bond would be when you spray another layer of gel coat on a tacky layer that hasnt been completely cured.

It is true you won't have a cross-linked bond but gel over gel can work so long as the final combined thickness is not to thick or the previous layer sanded close to the substrate. Unlike the substrate gelcoat is basically an unsupported resin and when you go to thick it will be more prone to cracking.

Again this is just my two cents based on my previous knowledge of how most paint substances work, and my experience of putting non flexing paint on a surface that may experience flex (very bad after all the hard work). If you want a thin layer then I recommend paint that needs to be re-done every few yrs, or put on a heavier layer of gel coat that will be less likely to crack.

Again, the thicker the application the more prone to crazing it will be..

 

[oldboat]

spraying gelcoat is not difficult. every old corvette that has been repaired correctly has had the gelcoat repaired/resprayed. check online for corvette gelcoat and you will many suppliers that have a spray able gelcoat.

 

[RichH]

Each gelcoat manufacturer has a schedule thickness of gel vs. the amount of flexure. They can also supply 'thickness gages' ... looks like a very teeny little 'comb' that you press into the wet surface to ascertain the applied depth, commercial automotive spray painting supplies also have these 'thickness gages' readily available. With such a gage you can easily control the amount of 'thickness'; and best of all you dont have to do the gel spraying 'all in one pass/application', and simply 'work up' to the max. thickness.

When spraying gel to affect a good surface cure ... you need to spray on a 'parting fluid' (polyvinyl alcohol, etc.) as the final coat to get a good 'cure' of the final surface, so that subsequent flat sanding and power polishing is rapid and doesnt 'clog' the grit with uncured resin. Such 'parting fluids' are the exact same that are first sprayed to the surface of a female mold where the gelcoat is sprayed-in 'first'.

 

[Maine Sail]

Each gelcoat manufacturer has a schedule thickness of gel vs. the amount of flexure. They can also supply 'thickness gages' ... looks like a very teeny little 'comb' that you press into the wet surface to ascertain the applied depth, commercial automotive spray painting supplies also have these 'thickness gages' readily available. With such a gage you can easily control the amount of 'thickness'; and best of all you dont have to do the gel spraying 'all in one pass/application', and simply 'work up' to the max. thickness.

When spraying gel to affect a good surface cure ... you need to spray on a 'parting fluid' (polyvinyl alcohol, etc.) as the final coat to get a good 'cure' of the final surface, so that subsequent flat sanding and power polishing is rapid and doesnt 'clog' the grit with uncured resin. Such 'parting fluids' are the exact same that are first sprayed to the surface of a female mold where the gelcoat is sprayed-in 'first'.

I've always used Wax-Sol mixed in with the final gelcoat application. Works well..

Wax-Sol

 

[RichH]

Sounds good, Ill try it next time. Better than spraying on the messy PVA, etc. anything to make 'life' less difficult - thanks for the tip.

 

[CanuckSailor]

An interesting thread - especially as I'm now painting my boat with Interlux Perfection for the second time. Happy with the results, but the idea of gelcoat has its attractions. A question here - can gelcoat be applied over a two part paint, should I go that route in a few years time? Second question - is gelcoat available in a screaming bright red?
Thanks for your expertise gentlemen.
(p.s. you can watch the process and comment at http://youcanpaintyourboat.blogspot.com)

 

[Barnacle Bill]

Oh my you peeps make everything so complicated. For touch up gelcoat applications all you need is gelcoat, gelcoat colorants to mix so that you can match your color, acetone, a Preval Sprayer, sandpaper, etc.

If you buy gelcoat with wax, you don't need PVA which is designed to be sprayed over gelcoat without wax so that it will block out the air so it will cure.

Blending the color can be tricky and some people are better at it than other, but after you are satisfied with the color, then put it some in a Preval Sprayer and catalyze it with MEKP (usually 10 to 12 drops per ounce or golf ball size) and then mix it up good and add acetone to thin it to a spraying consistency. Start with about 10% and then add a little more bit by bit until you get a good spray pattern (not lumpy) and don't thin too much or it will get runny and the color will not cover very well. After it cures, wet sand it starting with 320, then 400, then 600 and then buff it shiny with fiberglass rubbing compound.
The Preval Spray can be found at most automotive stores, or finer marine stores. It can be purchased whole which includes a power head and glass jar that screws on to the powerhead (or you can buy each part separately). It is too small for painting an entire boat, but you can do some larger areas, like a foot square, but it is great for small dings and scratches. Of course scratches have to be filled and sanded smooth with polyester products (do not use epoxy). If you have to do the whole hull, then I recommend painting instead of gelcoating. But, for spot repairs, my method works very well.
You want to spray over a sanded area (like 80 grit). The gelcoat will fill the sanding and you will get a good tooth. If you spray over a non sanded area, it will just peel off after a while. Also, clean your sanded area with acetone prior to spraying. You can also use styrene monomer for thinning, but acetone is more readily available.

 

[RichH]

Thinning exclusively with acetone will promote a 'weaker' gelcoat ... and with acetone you need to 'stand back' further to allow better 'flash off' of the acetone .... or you get 'pin holes' if the acetone 'flashes off' IN the spray deposited gel. The further you need to stand back to allow the acetone to flash the more gel gets carried away by wind/air and the 'rougher' the sprayed surface will result. Not a major problem, just be aware.

 

[wetass]

A question here - can gelcoat be applied over a two part paint, should I go that route in a few years time? Second question - is gelcoat available in a screaming bright red?
Thanks for your expertise gentlemen.
(p.s. you can watch the process and comment at http://youcanpaintyourboat.blogspot.com)

I luckily have never made the mistake of gelcoating over paint, but I saw someone who tried (They thought they were spraying gelcoat over gelcoat) - The yard had been hired to make a modification to the aft cockpit area on a rather large Hatteras and sprayed the ENTIRE rear 15 feet of the boat (inside the cockpit and out). Well, Hatteras' are painted (even the ones that have gelcoat from the mold) - I saw it the day after they sprayed and it all had to be sanded off. The gelcoat was all alligatored, wrinkled and shriveled up.

This obviously wasn't the best yard ;-( I still can't imagine how they couldn't tell it was painted as they had sanded and fiberglassed areas of that hull. And its not uncommon knowledge that Hatteras paints their boats (A yard should know this).