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Gel Coat problems

Discussion in 'Molding and Casting' started by Warren, Oct 30, 2006.

  1. Warren

    Warren Member

    I know this is probably me causing the problem, but I cannot get the hang of applying gelcoat to a plaster mold. Both times now the gel coat has not set up. The first time I thought maybe it was because I hadn't added enough activator, but the second time I doubled the amount. According to the can the gel coat has a working time of 15-25 minutes at 77 degrees and recommends doubling the amount if temp is below 65. In my case the temp was 70 yet the gelcoat was still fluid an hour later. When it finally did get tacky I applied the fiberglass resin and mat. After setting for several hours I checked and the fiberglass was hard with no tack. I separated the mold only to find that the gel coat in places was still wet on the outside where it contacted the mold. What am I doing wrong???

    Also, is the MEK activator that came with the gelcoat the same as the activator for the fiberglass resin? Can one be substituted for the other? The reason I asked is the tube that came with the gelcoat is not going to last at the rate I'm going. ;D

  2. Laurier

    Laurier Active Member

    it sounds like the catalyst is no good or not hot enough, or maybe a reaction to your release agent on the plaster.

  3. JL

    JL Taxidermist for 64 years

    Warren...your mold is not dry. Gelcoat as any fiberglass resin is highly affected by moisture and will not set up in the presence of moisture. Take a sample of that same gelcoat and activator and leave it in a paper mixing cup after you mix it.If the gelcoat is ok (as I suspect) it will become hard within the normal set-up time.And, yes the same MEK is used for both gelcoat and resin. Good luck...JL
  4. Kiwi

    Kiwi New Member

    JL is 100% right make sure your plaster is totally dry otherwise u will just have a sticky mess,been there done that and it is no fun....

    it may be touch dry on the serface but underneath there will still be moister, put it in the sun if you can, good luck!
  5. Warren

    Warren Member

    I appreciate the advice and will try once again with the suggestions provided. The plaster mold was allowed to dry for two weeks and appeared by all outward appearances to be dry -- it's weight was considerably less, and it was dry to touch, at least no dampness that I could detect. As for the release agent. I used shellac (2 coats) with drying time between each coat for a sealer, and then followed with two coats of Johnsons Floor Paste Wax over that, with each being allowed to dry prior to casting.

    Once again - Thanks
  6. newbirdman

    newbirdman Active Member

    Warren , I build boats so i know all about gel coat . First of all gel coat sets up ( hardens ) much faster than resin . 25 minutes is too long . It should set up in less then 10 . Now if you add too much hardner it wont harden properly and will be sticky . Yes the hardner for fiberglass is the same for gel coat , MEKP . It sounds likle your mold is dry enough . What about the extra gel coat you have left over in your mixing cup ? Is that hard and dry ? You should really be using PVA wax over the paste wax . I dont know what your making but why dont you make your our gel coat with body putty and resin . For some reason this mixture doesnt get effected by moisture . Rick
  7. Warren

    Warren Member

    The left over gelcoat was dry and hard, but it too took awhile to set up. It's a fish head (Redfish) I'm casting, by the way. I removed it from the mold last night, and the gelcoat was still not set up completely. Needless to say, this one is being chalked up as a learning experience. For my next attempt, I'm going to do like you suggest and used a thinned bondo/resin mixture for the initial coat, followed with the fiberglas mat and resin. I attempted the gelcoat, as I wanted to try a new approach and liked the white head you'd end up with. Well, maybe. ::) Thanks for the response, I appreciate it. Warren