Bad Gel Coat

[BoatManCave]

Perhaps someone there could explain why the gel coat did not stick to the hull.

The hull was prepped with Duratec VE gray primer* and sanded with 220 using a random orbital sander. I then rinsed the hull with a garden hose and large sponge. A week later I blew the hull off with compressed air and gave it a solvent wash with acetone and let it flash off for about ½ an hour. I sprayed gel coat* only on the transom (for a test) using a compressor HVLP gravity feed gun with a 2.3mm tip.

This was the schedule:

Batch Gel coat Duratec clear gloss additive* MekP thinner

1 4oz none 2.5cc none

2 4oz none 2.5cc none

3 4oz none 2.5cc none

4 8oz none 5cc none

5 6oz 2oz 5cc none

6 4oz 4oz 5cc none
_________________
Total Gel mix=36oz

I would guess that each 4oz took two or three coats on the 10 square foot transom. I cleaned the gun with acetone between each batch. The next day I dry sanded the transom down to 1200grit - it looked like a fine piece of white porcelain - I was happy. The total dry thickness was between 22-24 mils (I measured with a micrometer after it peeled off). - I was unhappy.

Before I knew that the gel coat on the transom could be peeled off with a putty knife, and about a week after I did the transom, I sprayed the starboard side of the hull (100 square feet). The prep was the same, and I used up about 2 ½ gallons of gel coat mix. The first 1 ½ gallons was a 4:1mix of gel coat/clear additive (4 parts gel coat to 1 part Duratec clear additive). The remaining gallon was a 50/50 mix. The batches were all 16 oz. The first couple of batches started to kick off in the gun as it was a very hot day and a slow spray - somewhat less than you would get from a spray can of paint, so in the following batches I reduced the MekP to between 1 and 1 ½% and added about 20% acetone (big mistake) to thin it - it sprayed wonderful.







The next day I went to admire the beautiful job and noticed that I missed removing a small piece of tape by the waterline. As I picked this tape off, I noticed that some of the gel coat seemed to be lifting from the hull. With my fingernails I could start removing pieces of gel coat which lifted easily from the hull. I then used a putty knife and could easily slip it under the gel coat removing larger pieces. The next half hour or so, I was using a 12 inch sheetrock blade and removing the gel coat in sheets as large as newspaper pages - it came off easier than any wallpaper I’ve removed from walls. It was when I got to the back of the boat that I tried to see how well the gel coat held to the transom. It held somewhat better, but I was still able to remove it all with a 3” blade.

The back of the gel coat was pure white; the hull was pure gray; little adhesion took place and there was no transfer of primer to gel coat or visa versa.

[Commuter Boats]

With the amount of acetone your thinning with, the primer could be soaking quite a bit of that from the gelcoat which in turn inhibits the bond. I wouldn't be comfortable with your prep procedure,I like a little more tooth than 220 ( maybe 120), how clean is your compressed air?,I don't know about your sponge [( was it clean?) I don't trust rages anymore because of fabric softeners ], you didn't describe your wiping procedure, did you use two towels, turning them often? Acetone is really good at moving contaminants around. Gelcoat manufactures really don't like more than a couple percent of acetone, a lot of people think MEK might be less destructive to the gelcoat then acetone. Styrene is safe to about 15% but can cause some yellowing and can reduce the hardness of little. The gelcoat people also prefer that you stay above 1.5% catalysts.
So maybe solvent inhibited, contaminated surface, minimum tooth, chemically altered gelcoat, minimal catalyst, sorry that doesn't narrow it down any.

[waikikin]

I've used duratec grey on a couple of plugs & wasn't aware of its use as a hull primer, it loves to polish & sands nice, a great foundation for the black high gloss top coat as slight wobbles can be tuned out & it all buffs up sweet, I dunno what the issue is as Commuter has indicated plenty of possibilities esp the solvent wipe & that grey primers love of self polishing with its own dust. Hawkeye industries has ( or used to) a pretty comprehensive application description for the products, better give it a good look. I've heard of a couple of "stick ups" when a molds created over it too early apparently due to solvent still in the substate- maybe thats what you need to get adhesion on the job your on. All the best from Jeff.