What's under my antifouling?

Barnacle Bill · Oct 23, 2017

The photo 1357 shows that you have blisters (osmosis) which is not unusual for a boat of your boat's age. To repair them, you need to have the bottom sandblasted or stripped and then a system like Interlux' Interprotect system epoxy water barrier coat applied. It doesn't matter what kind of antifouling paint was once applied because if you do the job, you will remove all existing paint anyway. After the epoxy is applied, then you need to apply the antifouling paint of your choice. In regards to your other photo. it sort of depends on where on the hull the scrape or sanding is located. It could be a coat or so of antifouling beneath the outer layer, but there's no way of telling what kind it is, if it is. If the area is close to the water line, then it could be the original color of the waterline stripe. When the boat is made, the hull is laid up with fiberglass matt, cloth, roving, etc. inside of a mold. Before this process begins, the shiny mold is sprayed with gelcoat in the color of your hull (I presume white). Where the waterline exists, it was taped off to define the stripe(s). After spraying the white hull color, the taping is removed and then they just spray the waterline area (which in this case might be blue). They don't spray a defined stripe, but the blue stripe is defined by the white gelcoat after the tape is removed and so they just spray the blue in a big swatch over the area. Later, if you sand through the white gelcoat, you then could reveal that blue gelcoat.

jibes138 · Oct 24, 2017

No need to reapply gel coat after you sand it. Just follow the directions for the interprotect or whatever product you use for barrier coating.

Crazy Dave Condon · Oct 24, 2017

Two things to consider. Any voids need to be filled and repaired first before any application. In addition you need to check the moisture level in the hull because if the hull is still wet, you need to hold off and let the hull air dry. It will depend on weather at this time of the year where you are at. Generally 60 days or longer and if in the north, hold off till spring unless the boat is inside and inside , that is another set of issues.

DayDreamer41 · Oct 24, 2017

I believe that you only need to sand down to the original gel coat, not down to the fiberglass, I am sure there will be more suggestions but from my research on this subject the blisters are caused by the type of original resins used, the hull will need to be stripped to the original gel coat and then coated with a barrier coating to prevent moisture intrusion.
http://www.smithandcompany.org/GRP/GRP.html

Head Sail · Oct 24, 2017

Pic 1357 looks like previous blisters that have already been repaired. Did you sand the antifoul paint off of them or did they look like that from the start? You may need to sand, fill any voids ( like the one in 1355), and reapply antifoul paint and call it good. Check with someone who does bottom paint for a living.

CloudDiver · Oct 24, 2017

@Suncatcher , Bill hit the nail on the head about the blue color, just two colors of gelcoat applied in the mold. Your blisters are the minor kind where the water molecules only penetrated the gelcoat and were between the layers, not deep into the laminate. You want to sand them down flush like you have and inspect for any other blisters that might be worse (into the laminate). Like others have mentioned above, no need to sand off all the gelcoat but you will want to use a fairing compound and longboard sand. I hope you are out of the water for the winter, this is an involved process. You can finish the sanding now but you will want to wait until spring to apply fairing compound and paint. For the immediate weeks, get your sanding done as much as possible and have the moisture levels checked. If she sits for the winter having the antifoul sanded off any those minor blisters sanded out she will dry out, but check moisture levels again in the spring. You can mix your own epoxy fairing compound using fairing filler mixed to a tick paste in epoxy or use the 3M gallon cans that are premixed (just add hardener cream). After long boarding you can apply the Interlux epoxy barrier coat, it will take 3 gallons to coat your hull, rudder and keel. Use 1/4" nap rollers, not foam... foam will fall apart and does not build film thickness well. Be ready to hot-coat your antifoul.

DayDreamer41 · Oct 24, 2017

You apply the antifouling before the barrier coat drys

CloudDiver · Oct 24, 2017

DayDreamer41 said:
You apply the antifouling before the barrier coat drys

Basically that is correct, but be more specific. Hot Coating is rolling on the anti-foul before the epoxy barrier coat is totally CURED, but it is dry to the touch. In 70 degrees the last coat of epoxy barrier coat should kick and be dry for the knuckle test in about 45 min to 3 hours (direct sun gives you the low 45 min end, shady side of hull in 1 to 2 hours, humid climates you should go 3 hours). Place your knuckle lightly on the coating and you may leave a light skin print but no paint should come off on your knuckle. Don't use your finger tip because of the oils.
Interlux Interprotect 2000E can be coated over itself up to six months between coats without sanding, but after the final coat they recommend hot-coating the anti-foul for best adhesion. Noteworthy; I've never heard anyone complain that their first coat of antifoul did not adhere well because they didn't hot coat, but they did apply the first coat of anti-foul the same day or by the next morning. The window for hot-coating with epoxy based paints is usually several hours unless it is ridiculously hot and dry. So technically, in 70 degrees F, if you finish the last coat of Interprotect in the evening and you apply the first coat of antifoul the next morning this can still be considered hot-coating (be careful of moisture from morning dew).

When doing a barrier coat followed by anti-foul, you must have a schedule (plan) and stick to it. This is especially critical for anti-foul paints that degrade in open air while you are waiting for the travel lift to get you back in the water.

CloudDiver · Oct 24, 2017

Suncatcher said:
I am in Canada...so my boat will be on the hard over winter.
Thx for the hot coating info. The bottom will be sanded and left to dry over th winter.

Since the very small blisters that I have uncovered never seem to invade the fibreglass, rather than sand them down i.e. the gelcoat, can I just put the the Interprotect over them? Then the bottom will have the Interprotect as well as the gelcoat.

You will still want to apply epoxy based fairing compound and do some long-boarding, especially on that area you pictured where you went through the blue to the laminate. If you don't at least make an attempt to do some fairing you will see all of that through the paint and your hull will look 'lumpy'. Don't be afraid of long boarding, its therapeutic... trust me!

CloudDiver · Oct 26, 2017

On longboards, 'Long' is the key word. They also need to be slightly flexible to conform to hull shape. Usually they will incorporate a handle at one end and a knob at the other for grip; this is important for comfort, maintaining positive contact, and conserving energy through the continuous strokes.

This is inexpensive and not considered a longboard, it is a long 'block', but it is affordable, a great product, and at 24 inches it will work for this application. I used to use these automotive type blocks when sanding and wet sanding varnish on bright finished canoes and Adirondack Guide boats;
https://www.amazon.com/Dura-Block-A...coding=UTF8&psc=1&refRID=BV05A41NBR0PHSP2BEZK

This one has handles but its shorter, 16 inch; (I think they have this one on Jamestown Distributors for much more money)
https://www.amazon.com/dp/B00ZRVL9B...bbd7-29a393cfe2ad&ie=UTF8&qid=1509034105&sr=1

Another one that is only 16 inches but has better handles. On each one of these note how the sand paper attaches;
https://www.amazon.com/dp/B0025DOD9O/ref=psdc_15707211_t4_B000RFRR4U

None of these are the 36 to 48 or even 72 inch long boards that have 'suspension systems', require two people to operate, and cost upwards of $400. Those types of boards are used in new custom boat construction where a team of 4 body guys get the fairing perfect prior to paint. For sanding bottom paint, repairing blisters, and smoothing out some 'oops' spots, I think a 16 inch board will do fine although my preference would be 24 to 32 inches. I usually get long belt sander belts I buy from an industrial overstock store for cheap, cut them, then use spray adhesive to attach them to a strip of 1/4" MDF. About shoulder width is what you can operate with just one person so for me about 30 inches. I use regular cabinet knobs as the handles and don't put them all the way at the ends, maybe 4 inches in. Anyway, I know you aren't making your own board so I'd go with something similar to the above links.

Suncatcher said:
I am planning to using fairing compound and have been reviewing longboards online. I don't have the facilities to make one. Any recommendation especially regarding length? I have an area on the hull too that I will need to repair abour 12 inches diameter.

Thx

What's under my antifouling?

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