Spraying gelcoat questions

[captronr]

I'm not ready to try this yet, but my blue hull Eclipse has a lot of gelcoat rash and scratches on one side.  I have the blue gelcoat from Compac, and know I need to get some MEKP for hardener. 

I have searched a lot online and this site for how to repair gelcoat.  What I'm thinking of trying is to lightly sand the rash areas, and wipe down with MEK.

Then I have a small (like 4 oz) air spray gun.  Would like to mix a few ounces and spray it on after masking off other areas.

For the price of the gun, and effort involved, I'd trash the gun and buy a replacement as needed.

What am I missing with this approach? 

Thanks.
Ron

[carry-on]

Lots of info on YouTube and at Preval.com. Recommend the YouTube/s by Tap Plastics.

[bruce]

I use spray guns at times in my work, although never with polyester. I did find a void in the centerboard trunk cover on my Picnic Cat which I was able to fill and blend very nicely.

Not sure how big an area you're talking, but an airbrush with the largest nozzle was suitable for what I was doing. After filling and leveling the void with epoxy and glass to 220G, I thinned the gel coat I got from Com-Pac to a sprayable consistency. I don't recall how much styrene I used, but it was in excess of what was recommended. Test everything off the boat first to be sure you're OK. After masking, I sprayed on enough to cover the fill, let it set up, and sanded it down with 400G. I needed to spray a bit more where it was thin, I probably did that the next day. Leveled with 400G again, and rubbed it out with a maroon ScotchBrite pad and Novus #3. Couldn't make it out, even in person.

Taping to a hard edge is best, of course, but in my case I feathered out the blend fore and aft. The sanding took care of any dry spray at the margins. I used a good airbrush, that I didn't want to trash, and made sure I broke it down and cleaned it thoroughly before the polyester set. No bad surprises. I might not use my best Iwata gun, but I also wouldn't want to have to deal with a bad spray up from a cheap gun.

My boat dry sails out of my garage, so the gelcoat was in great condition. The gelcoat Com-Pc provided was a little light in color, but with time and grime it matches perfectly. An oxidized gelcoat won't be as easy, especially in a dark color. Good luck!

[captronr]

Thanks guys!

Part of my issue is terminology.   I thought the hardener you used was MEKP with the gelcoat was MEKP.  Is that the same as styrene?

My boat came to me with two large dock rash spots, and then I added a few long scratches on my own.  So I have two larger spots that I could spray, and probably could brush the other scratches.   

I will consider getting an air brush, instead of the cheapy cup gun...............

Ron

[bruce]

You're right, you need to use MEK peroxide as a hardener, 1-2% based on temp. etc. Test first, as always. If you're afraid of using up your color-matched Com-Pac gelcoat in the testing, get some generic commercial stuff to play with.

Styrene is a thinner, these guys say use 5%. That would have been to thick for my Badger Crescendo airbrush, even with the big nozzle, so I thinned more, maybe 10-20%. With more thinner, you'll get more shrinkage as the solvent evaporates. Too much, or the wrong type of thinner, and you could get some defects, like cracking.
https://www.youtube.com/watch?v=6F0j5OgVAOI

If you've got a gun already, try it. I never use a Preval sprayer, but people do well with them. An airbrush is typically used with thinner medium, so as useful as they can be for blending, I wouldn't buy one to spray polyester. A small HVLP touch up gun would probably be best for this. They can handle thicker mediums, and small ones can work with standard compressors. They use a lot of air.

If you can brush in enough to build up long scratches, that could work. The problem with trying to level a built-up area is the tendency to dish the surrounding surface. In my case, the existing gelcoat was less than 1/32", it was possible to sand through if I wasn't paying attention.

However you apply the gelcoat, if you can level it satisfactorily you'll be OK. I think your bigger challenge will be matching the gloss and color. The new gelcoat will probably be darker and more saturated. Try rubbing out with coarser compounds and don't try and go too bright. That will diffuse the color some.