Re-gelcoating

New to the forum but I am days away from trying my hand at RE-GELCOATING a butt ugly Hobie Cat 14 circa 1974. Had some soft spots on top of the hulls which I have injected with resin....they are rock hard now. Made a repair on the side of one hull. Have sanded the hulls with 60 and then 80 grit.

Heres what I have purchased:
-Harbor Freight HVLP spray gun
-1 Gallon of Sea Hawk brite white gelcoat with catalyst and wax
-2 Qts Sea Hawk MEK Gelcoat Thinner

MY INTENT:
-Wipe down hulls with Acetone
-Mix gelcoat with 5% MEK....No wax....then mix in 2% catalyst
-Spray light first coat
-Wait one hour (MFG Sugg)
-Spray slightly heavier second coat
-Wait one hour
-Spray last heavy wet coat with wax additive

QUESTION?
-Does anybody see any problems with my plan?
-With a high quality HVLP spray gun of this caliber...lol...should I thin at all?
-Will more thinner mean a more smooth finish...(less sanding)?
-Do I need to sand with 220? Not too worried about a boat show finish...just want a good durable BRITE white finish.
-How hard is it going to be to go white over yellow?....ok I mean brown.
-Is a gallon of gelcoat enough?....I know 48 SQ ft to a gallon but is that 3 coats?
BTW I am not using Duratech

Thoughts and suggestions??????

 

I would have just used a good paint to tidy it up.

 

Thick gel coat is not a good idea !!Oh sure wil be nice to look at but later could start to mud crack and chip very easly !! so just be aware !!
Me i would use a good 2 pot urethane and a couple of coats ! a good shine and Urethane can be harder than gel and theres no fears of the dreaded mud cracking! easyer to touch up dings and scratchs that will happen for sure ! .But thats me !! stuff gel coat !!

 

Gel coat is a lousy paint. It will also leave a lumpy surface that must be cut back with progresively finer grades of sandpaper then buffed or compounded before you get a smooth shiny surface. Go with a two pack PU or urethane acrylic top coat preceded by fairing, filling so that any pin holes exposed when removing the gelcoat have been filled or bridged. This will be the most time consuming part of the process. The HVLP gun will also be better adapted to spraying paint than it will be to shooting viscous gelcoat. I also think gelcoat or polyester resin is best thinned with up to 10% styrene not MEK or any other solvent for that matter.

Dino

 

Acetone or styrene to thin with ??

100% I would go for acetone over styrene everytime !! But that just me !!
Acetone will evaporate very quickly after it leaves the end of the gun and by the time its hit the surface 50% has gone and if you spray fine coats 99% is gone in a very short time!!
I even use Acetaone for urethane paints as well !!But check first that its compatable and will mix with the product you are using !!

Styrene on the other hand is slow to evaporate so its slower to do a spray job of multiple coats , its also inclined to cause yellowing if used in Gelcoats out in the sun for long periods of time . It stinks and is hard on you skin !! sorry cant think of anything good to say about it !!
New zealand has a exceptionally high uv sun shine and styrene is almost a nono to use in gel coats for repairs etc

MEK !! sorry wont go there !!, end of story !!.

 

I Re gelcoated my 38 year old Hobie 16 2 years ago and it has held up just fine. This was my method,
1, Sand old gelcoat with 40 grit
2, Spray first layer of gelcoat without wax,
3, Wait for first layer to tack off, then spray second layer of gelcoat without wax,
4, Wait for second layer to tack off, then spray third layer with "patch booster" which thins the gelcoat and already has wax in it also.

The patch booster gave a very good finish for gelcoat, not a 2 pac finish but much better than straight gelcoat orange peel effect.

5, Let set then sanded down through the grades, 400, 600, 800, 1000, 1200 then buff, buff, buff and polish 5 times. This gave a mirror finish, I used Vinylester Tooling Gelcoat in bright red, best looking Hobie 16 around I'm told

220 grit will give you a smooth finish, but not really a bright surface, once you've knocked the orange peel off the gelcoat the next layers get easier

I saw another Hobie 16 that repainted same time as me with 2 pac PU, I have to disagree with the others recomending this method, His hull bottoms have not held up at all to the beach landings and general wear of sailing and are scratched and not looking great, where as my gelcoat hulls look just about as good as when i did them, and they certainly get a killing on the beaches round here I misjudged a channel depth and sailed over about 100 metres of rough gravel of about 100 mm (4 inches for those metrically challenged) depth, rudders got a bit mashed but the hulls just needed a buff and polish and good as new

Sorry for the novel ^^^^
Hope this helps DAVE

 

Styrene is polymerized while solvents evaporate.

Dino

 

40 grit ? ille bet there is a zero missing there.

 

I would go with a small amount of acetone if you don't have a patchaid type product that is supplied by the manufacturer. All thinners have a down side, so use as little as possible. Be careful when adding any type of thinner, they can change the color and look of the finished job, so if you thin only the last coat it's possible that when you start sanding and go through it you may see a different color or look in the layers under it.

People frequently use various products to thin gel, some at very high ratios, one problem with this is if the thinner used doesn't evaporate it will reduce the pigment loading of the gel coat and gel coat relies almost 100% on the pigments for UV resistance. If we could reduce the pigment loading by even 2% and get the same weather resistance we would, pigment is expensive, but we can easily see even a small reduction of pigment in the testing.

 

Thanx for the encouragement Dave!
Suprised at the responses recommending Acetone.....Sea Hawk only recommends their MEK thinner or Patch Aid as as thinning agents....so thats what I bought. Gonna give it a whirl at 5% and see how it does on first coat. Looking at my spray gun and noiticed it has a 1.4mm nozzle.....thinking it's gonna be hard to get a heavy coat but we'll see.

 

There are only a few manufacturers of gel coat, but there are quit a few small distributors that buy and repackage it with their name on it. There are also several different solvents or monomers that can be used to reduce the viscosity of gel coat. Small amounts of these may not have much of an affect, but whatever the affect is it does not enhance any of the physical properties, including weathering, and the more you add the bigger and more evident the affect becomes. Poor cure, poor weathering, cracking, chalking, blistering are all more likely when more than a small amount of anything is added. The only thing that is improved is the leveling, so sanding out the orange peel is quicker and easier.

 

Thanx Ondarvr...
Makes sense. Thinking I am going to try a few experiments......Sea Hawk yacht finishes even recommends starting at a lesser amount of thinner and working your way to 5 or 10% at the most.

 

Pic

Here is a picture of the initial project as purchased for 100 bucks .

 

Thinning gel coat is to be able to get it to come out of a spray gun so it has a consistancy of paint !As for reducing the pigment no its only the thinners that evaporates but the pigment settles faster so the surface gets the lighter materials that the gel coat is made of coming to the surface . the pigment is in a differant part of the sprayed film thickness its not lost or reduced !!. multipul fine coats is better than one thick coat to solve this problem !!

 

You have misunderstood what I said slightly, and pigment float is an issue.

The added solvent and/or monomer does improve the sprayability of the gel coat, the goal is to have it level better though, how it comes out of the tip isn't as important as how it levels. It just happens that if you improve one aspect the other typically improves as well. Acetone can make it spray very well, but if too much is lost on the way to the surface it doesn't improve the leveling as much, it can be dry and lumpy. This can be an issue with gel coats containing large amounts of MMA (used in place of styrene for better weathering), it evaporates more like acetone and in hot, low humidity regions can cause a dry spray affect.

I said if the thinner doesn't evaporate it reduces the pigment loading, with acetone you are hoping it evaporates, small amounts typically do, larger amounts can result in an under cured and soft film due to trapped solvent. Yes, the color can change from pigment float, I didn't go into every detail of why it may change color, only to watch out for it when adding thinners, and the more acetone you add, the more of a chance there is for it to happen. Why it changes color isn't as important as knowing that it can.

The pigment loading issue comes into play when using styrene, MEK, toluene, patch boosters and other things that don’t evaporate.