Bad Gel Coat

Discussion in 'Fiberglass and Composite Boat Building' started by BoatManCave, Aug 21, 2009.

  1. BoatManCave
    Joined: Aug 2009
    Posts: 1
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    Location: los angeles

    BoatManCave New Member

    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.
  2. Commuter Boats
    Joined: Oct 2006
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    Location: Southeast Alaska

    Commuter Boats 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.
  3. waikikin
    Joined: Jan 2006
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    Location: Australia

    waikikin Senior Member

    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.
  4. sdowney717
    Joined: Nov 2010
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    Location: Newport News VA

    sdowney717 Senior Member

  5. ondarvr
    Joined: Dec 2005
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    Location: Monroe WA

    ondarvr Senior Member

    This is an old thread, but may be useful to others.

    It's not a mold release primer. He used the VE primer, not the surface primer.

    While the surface primer version can be polished and used as a mold finish if a super high gloss isn't required, or a high number of parts aren't going to be made, it's still a primer and can be used in that way.

    All of the primers can be used as a mold surface, they just don't achieve a high gloss. One version was formulated to have a higher gloss if the person intended to use it as a temporary surface.

    220 grit wouldn't be a good choice when gel coat is going to be a sprayed over it. 80-120 grit would be better.

    Cleaning with water can be OK, but frequently water can have a lot of minerals and other stuff in it. Hard water can leave a scum on the surface.

    Acetone is a very poor solvent to use for cleaning a surface prior to painting. While it may dissolve any gunk on the surface, it evaporates so quickly that it leaves all or most of the gunk behind when it does. Only now you have the contaminant spread over the entire surface.

    Compressors without a trap and filter can blow oil and/or dirty water on the surface.

    In places he referred to "MEKP thinner", not sure what that was about.

    Thinning with acetone isn't the best option, and 10% is a lot.
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