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Author Topic:   thinning gelcoat for preval sprayer
boatdryver posted 03-31-2006 11:29 AM ET (US)   Profile for boatdryver   Send Email to boatdryver  
I'm about ready to start spraying Spectrum gelcoat over multiple small (1-2inch) repairs inside which have been filled with marine tex and sanded flush with #240 grit.

Previous posts have confirmed that the quart liquid needs thinning for good results with a Preval sprayer. Some advocate using acetone, others say use styrene to thin. I've done a lot of spraying with spray cans and get fine results with ordinary paint. I've never sprayed this thick gel coat. The surface sealant and hardener and instructions have been ordered

I have two questions:
What is the correct thinner?
Should the marine tex filler be sanded to #1200 grit BEFORE coating with gelcoat?



BBS posted 03-31-2006 12:06 PM ET (US)     Profile for BBS  Send Email to BBS     
I used acetone as I could not find styrene, which is the recommended thinner. Regardless, it turned out great. Those little sprayers do an excellent job for small repairs.
The Judge posted 03-31-2006 12:27 PM ET (US)     Profile for The Judge  Send Email to The Judge     
Make sure you have more than one sprayer and you don't put too much hardener in it. Once that gel kicks, sprayer is done. If it kicks too fast it can burn your hand and if dropped may explode(no BS). I personally found usuing a foam brush and brushing it on works better(no orange peel).
boatdryver posted 03-31-2006 06:42 PM ET (US)     Profile for boatdryver  Send Email to boatdryver     
OK thanks for that. I used a foam brush once to brush awlgrip on some anodized aluminum window trim and it worked great. I was surprised how the vapors made me feel a little sick holding a 2 ounce cup and dipping the brush in it for only about 15 minutes.

Now how about the question of how fine to sand the filler before applying the gel coat. stop at 240 grit to give the gelcoat something to grip or go on to 600 or higher to give a smoother final finish?


rtk posted 03-31-2006 11:23 PM ET (US)     Profile for rtk  Send Email to rtk     
I have had good results sanding to 220 grit, then spraying the gelcoat, when doing a spot spray for a repair. You are only trying to rough the surface so the gelcoat will adhere via a mechanical, or secondary bond.

For a repair that size, I like to sand an inch or two of the gelcoat outside of the filled area. Spray the gelcoat close to the edge of the sanded area, but not to far beyond the sanded area.

If you spray the gelcoat over the edge of the repair onto the unsanded gelcoat, over time the edges of the gelcoat will lift, and sometimes you will even be able to see a "ring" outlining the edge where the gelcoat and unsanded substrate is.

To finish and blend the spray job, start with some 400 grit on the edges, and lighty feather where the sprayed gelcoat meets the surface. After just getting rid of the "edge", then work your way up to the higher grits until you are ready to compound out the spray patch. Wet sand for this process.

If I remember, the Preval was around 5-6 ounces? I always used acetone, around 1/2 - 1 ounce per 5 ounces gelcoat. Temperature, humidity and gelcoat viscosity will dictate how much thinner is needed. Before you attempt the finish coat, play with thinner amount and hardener amounts on a scrap or smooth surface. Take note of how each sample looked when sprayed and how long and well it hardened. Trial and error will get you dialed in on how much hardener and thinner you need. I think I used to use 10-20 drops of hardener per ounce of gelcoat. Temperature plays a big part regarding how much you use.


boatdryver posted 04-01-2006 11:32 AM ET (US)     Profile for boatdryver  Send Email to boatdryver     
Thanks, Rich. that's exactly the kind of advice I needed. If I wait till the temp is 70 I'll be into the boating season so I'm stuck with doing it at 55-60 degrees, which will require trial and error.
Tonym posted 04-01-2006 12:00 PM ET (US)     Profile for Tonym  Send Email to Tonym     
I purchased a double "contractors lights" at home depot.
I think they put out about 3000 watts total. you can pre-heat small areas then spray and then keep them on for a 1/2 an hour till the coating has set good. I found them very useful on cold days. Just don't them too close to the part.
Good luck, Tonym

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