Spraying Bottom Paint

[Ted]

I'd like to touch up a few small areas of bottom paint where the paint has thinned. I'm using Interlux Micron Extra. Has anyone had experience spraying bottom paint using a Preval spray system. I'm specifically interested in the Preval system and not a compressor/spray gun application. The job is too small to bring out the compressor. My bottom has been wet sanded to a 400 grit finish and I would like to keep it as smooth as possible. Any success/failure stories to share?

 

[sailormanbigd]

wow, never heard of spraying bottom paint

hey don't know if you can spray it or not? My question is why would you want to? Bottom paint is not glossy or anything. If you only have to spot paint then why don't you just use a foam roller cover and a good bristle brush and be done with it? I am not an exspert but i would think 400 gritt is too fine a surface for good adhesion with the paint? Also, i was under the immpression that a very smooth bottom is not the fastest surface, something about breaking surface tension with a coarse finish? someone will likely clear-up this question. get-er-done and go sailing!

 

[Alan]

Micron Extra is an ablative paint. The leading edges of the keel rudder and bow always wear a little faster but the rest of the paint must very fairly thin also. That being the case if you're going to do this (which it sounds like you need to) then do the whole job properly. If you spray Micron you will need to cover all boats near you, wear protective clothing and you MUST wear a respirator. The solvents in the paint are HIGHLY toxic and are absorbed through the skin as well as your lungs.
I have sprayed the bottom of my boat for years but have switched from this highly toxic stuff to a water based and very effective alternative. Preparation of the surface is the most important part of the job. 150 grit sanding is ideal and then the entire surface must be washed down and dried before painting. Again, the wash down for Micron is a toxic solvent like Xylene (cancer causing very nasty stuff) and requires that respirator again. Washdown with a water based paint is simply water.
Now you are ready to spray using multiple thin passes till the desired thickness of paint is achieved.

 

[tommays]

If you use there brushing thinner it will brush on smooth and you could take a shot at spraying BUT that would be well above the thinner level they want you to use

 

[RichH]

You can just as easily 'Trowel' it on as spray it.
For small areas just use a 4" thin foam roller to establish a base coat, when that almost 'cures' knock down the roller pimples with a hard polyethylene trowel (the yellow WEST trowels are good) and let fully cure. When troweling .. only one 'stroke' with a trowel, subsequent 'strokes' with a trowel will create a rough surface. Then after a few days, fill the irregularities with more roller applied paint and 'knocked flat with a trowel' using the hardened peaks / 'roller pimple tops' of the first application as the 'guide' of the troweling. You may need to apply a third coat but will soon arrive at a surface finish as smooth as a babys ass ... just like as with a spray gun.

For a 'small' job, using a Preval will be the easiest and 'less mess' to get a SMOOTH surface.
The difficulty with a Preval is the thick highly viscous bottom paint - dont thin 'extra' by more than 10% (using 216/433, etc.). If you do use a Preval, remove the teeny 'strainer' on the Preval's suction tube which easily becomes blocked and 'pre-strain' the paint using a conical paper/mesh strainer - from an auto paint supply. With the suction tube strainer removed, and the paint 'pre-strained' you will get better flow (ditto when spraying gelcoat with a Preval).
Simply spray on a windless day, hold your breath when spraying and stay on the 'up-wind' side or to the side of the flowing air. Just hold your breath when spraying and use a heavy 'dust mask' so you dont aspirate the 'liquid aerosols'.

Hull flow turbulence (parasitic drag) from a rough surface is especially egregious from all 'leading edges' and for about 10+% LOA (cord) back from the leading edges ... if they are not absolutely flat and smooth / fair. Therefore concentrate on the leading edges and at least '10% back' from the leading edges.

hope this helps.

 

[finding41]

I am working on the same idea of a golf ball. Lots of dimples for better water flow. Just disrupt the surface relatively evenly and ta da. I'll test my theory by sailing it like this this year and finishing the bottom for next year. Then compare top speeds.
Here is a picture of my bottom. If you look closeley you can see the attention to detail.

 

[arf145]

Nicely done, finding41! Hard to simulate that natural uneven surface but you've managed it.

 

[Ted]

Just wanted to report back about spraying bottom paint with a Preval sprayer. I touched up a few areas this weekend with fantastic results. I thinned the Interlux Micron Extra with 10% of #216, the recommended spraying thinner. The paint was put through a paper filter strainer which assured there would be no lumps. As Rich suggested, I removed the strainer at the bottom of the pick-up tube and the paint flowed perfectly the entire time. It came out smoother than the rest of the bottom that had been wet sanded with 400 grit paper. It was easy to feather in the touched up areas to the existing, previously painted surface.

 

[catsailor]

Someone is going to have to explain to me, in the real world with a soft paint like Micron and with slime growing on the boat over the next 2 weeks, what difference 400 mesh wet sanding makes. Unless the boat is dry sailed, I'm sorry, no difference at all.

When I worried about sailing fast I had Nomex/epoxy multiull; that worked really well, over 20 knots with a--gasp--rolled bottom.

 

[Nodak7]

Someone is going to have to explain to me, in the real world with a soft paint like Micron and with slime growing on the boat over the next 2 weeks, what difference 400 mesh wet sanding makes. Unless the boat is dry sailed, I'm sorry, no difference at all.

When I worried about sailing fast I had Nomex/epoxy multiull; that worked really well, over 20 knots with a--gasp--rolled bottom.

Catsailor....I wish there were a "Like" button for your post here! "Gasp"! Because I would have hit it!

 

[RichH]

Someone is going to have to explain to me, in the real world with a soft paint like Micron and with slime growing on the boat over the next 2 weeks, what difference 400 mesh wet sanding makes. Unless the boat is dry sailed, I'm sorry, no difference at all.

When I worried about sailing fast I had Nomex/epoxy multiull; that worked really well, over 20 knots with a--gasp--rolled bottom.

The flatter the paint the 'faster' /easier the slime release.
The flatter the paint the longer the paint will last .... less absolute surface exposed.
The flatter the paint the less ablative release because of the reduced turbulence AT the surface.

Just calculate the Reynolds Numbers based on 'surface roughness' .... any tech. pipe/tube data book will have the 'numbers' .... then just estimate the immersed wetted area and 'crank the numbers' ..... eye-opening even on a newtonian F=ma basis or pure energy basis. Commercial long distance ships get up to 15% better 'fuel economy'.

 

[BucekS87]

hey don't know if you can spray it or not? My question is why would you want to? Bottom paint is not glossy or anything. If you only have to spot paint then why don't you just use a foam roller cover and a good bristle brush and be done with it? I am not an exspert but i would think 400 gritt is too fine a surface for good adhesion with the paint? Also, i was under the immpression that a very smooth bottom is not the fastest surface, something about breaking surface tension with a coarse finish? someone will likely clear-up this question. get-er-done and go sailing!

I agree with you..

 

[RichH]

Here's some tech data from the USNA hydrodynamic tow tank (Annapolis) ... and you need to approach a surface roughness of 130µM to optimize hull roughness for normal sailboat speeds. http://www.dtic.mil/cgi-bin/GetTRDoc?AD=ADA424978
These tests are for plates towed where their surfaces are parallel to direction of motion. In this report, no tests were done/recorded for the viscous drag from leading edges.

130µM is one hell of a lot 'smoother' than what one can 'slop on' with a 'paint roller'.
220-320 grit would be better than 400 grit. ;-)

In the 1987 Americas Cup, Dennis Conner applied "rivulets' (Boeing Aircraft)... an embossed plastic sheeting to the underwater surfaces .... these had a surface roughness profile of ~130µM - 150µM ... not some globbed-on 'pimples' and 'wads' from a paint roller.

 

[finding41]

In all seriousness I did hope that the dimples on my bottom would make it faster. I think they do! I understand the theory of the smother it is the less drag, but my hull (like most of the other non racing sail boats) is a displacing hull. So Its going to push water no matter how I force it through the water. Not plane.
Back to the golf ball. Why does it have dimples? If it didn't have them it wouldn't fly as smoothly in the air. Here is the similarity. If my hull is dimpled like a a golf ball it will allow for smoother transfer of the hull through the water. Why? Because there will be a thin cushion of water between the hull and the slower water around it.
I'm not going to get out the drill and test my hypothesis on my boat. So I goggled it and like any lazy guy that's up too late I skimmed the debate in the attached page and determined. I think I'm on to something.
http://www.storerboatplans.com/wp/d...d-a-golfball-dimple-finish-help-it-go-faster/
I just rememberd the last summer Olympic games. The swimmers or some of them had dimpled swim suites.... I wonder what this years games will bring? Dimpled Lazer hulls?

 

[xcyz]

The boys on Mythbusters tried this with a Ford Taurus. I would think the dynamics are the same whether in or out of water.

http://videos.howstuffworks.com/discovery/39251-mythbusters-dimpled-car-minimyth-video.htm