Stupid question-Glass to gelcoat bond?

After applying gel coat to a mold how is the first layer of glass bonded to the gel coat?
Do you let the gel coat completely cure and then rough sand to get a mechanical bond?
I assumed there had to be a chemical bond or it could delaminate but if the first layer of glass is laid up with the gel coat uncured I would think it could print through.
I should know this but I've really only done structural repairs, not much cosmetic.

Thanks

 

As I understand it:
Unwaxed gelcoat is inhibited in its surface cure, by being exposed to air, so the next layup will have a chemical bond to it.There is an ideal "window" of time to get the best bond , depending on your conditions.(Temperature humidity etc)
Basically ,that is the convenience of polyesters.Epoxy the window is only a few hours. Poly is days.
If you second coat too soon you will get styrene attack to the gelcoat, or your second layup is too coarse or gelcoat too thin you may get print thru.Tissue over gelcoat gives good results. I prefer epoxy, but it is more work.

 

Normally the gel coat is ready to laminate on in a couple of hours or less and it's recommended that the first laminte is applied as soon as it's ready. If you can touch it with your finger and nothing comes off on it, then it should be ready. You don't need to prep the surface, just laminate on it, at this point the bond will be as good as possible, as it continues to cure the bond may not be as good. Most of time you will never notice the difference, but it can be seen in lab testing. As the gel coat sits with no protection the surface can become contaminated by dust dirt and other stuff floating around in the shop, this can cause more problems. While not recommended, gel coat frequently is left in the mold for a couple of days, as in over a weekend with no detectable problems.

 

You have to be sure you are using unwaxed gelcoat when you put it in a mold to be laminated over. If you laminate without the gelcoat cured (or thick enough) you will have bigger problems than print through. It will wrinkle up into a mess called "alligatoring" when you start laminating. Styrene settles and inhibits curing, so if you have a mold where it will settle and collect, like most boat molds, it's best to run a fan to blow it out and allow the gel to cure if you are trying to laminate within a few hours.

 

make sure the gel is unwaxed, if u spray make sure its spraying gel, if your brushing use brushing gel which is thicker and wont sag as much, you will want to lay up the glass when the gel is hard, it will always look shiney and wet and be a little tacky, the ideal is to laminate right over with zero prep i have seen gelcoat left in a mould for over a week and all was good, learnig to apply gel is a little tricker than just painting a mould, if the mould is not prep right the gel will stick , also you will want to wait a few hours after waxing as the gases from the wax will cause many problems, if the gelcoat is to thin the resin will wrinkle the gel and if its to thick it will prerelease this some times only happens during the layup of glass the frist layer is called the skin,and is to late to fix. use one layer of glass and remove all air bubbles, any air bubbles that are left will often burst open in the hot sun , if you have tight corners to go arounfd you can use a little paste to help with the lay down of the glass, i use some gel mixed with milled fibre and cabosill dont use to much just a little works,
Also you may want to paint the gel twice to advoid thin areas, you must make the gel a little hotter that the frist coat to avoid the first layer to wrinkle, i like to add a little resin say 10% make the gel flow smooth, remember also to use a fan if the mould is deep the styrene is hever than air and needs to be pushed away to let the gel cure if its a small mould you may be able to turn it upside down to let the styrene escape, and wear a sweatband u dont wat any water droping while you gel

 

On split molds I like to put an extra layer of gelcoat over the split in the mold, so when I'm dressing the split line on the part, there is enough there so I don't go through the gelcoat.

What are some of the problems you can get from the wax offgassing?

 

Tja

Hello, If you can use a ball point pen on the gelcoat it's ready for layup.

 

I appreciate all the info.
So I shouldn’t worry about applying gel in the evening and then lay-up the skin early the next mourning?

Any other tips on this would be great.

 

Normally you don't wait more than a few minutes between coats and the total build of 18 to 20 mils is applied in 3 passes, you can go back after it's hard and brush gel coat on any thin areas.

You need to use a pressure pot, not just an automotive siphon type, a siphon gun is designed for very thin paints and won't flow enough gel coat. You can get a cheap pressure pot at Harbor Freight, they work OK for limited use.

If the part is small you can use a brush to apply it, you just need to make sure you get the mold covered completely with no thin areas. When brushing I let the first coat get hard then apply a second coat.

 

if you use 2.0 OR 2.2 TIP ON A SPRAYGUN you should be ok you may want to thin the gel with 10% acetone you may want to spray with more presure than the gun says remember this is gel not paint, if you spray 3 coats with a basket weave patern you should be ok .you want a min of 14 mil and a max of 22. if you spray fast with your passes you will get on less and may need up to 5 passes to get the right thickness, get a mill gauge which you place into the wet gel and it will tell you your mill takes the guess work out

 

We build fast RIBs - see www.explorermarine.co.uk - Here in the UK the temperature opf our workshops varies a good deal, so the gel layers are judged by the "squeak ruke" If you run your finger across the gel surface and it squeaks it is ready for another layer or for a backup layer of 300gram / metre sq. mat.
We usually apply two brush gel layers - three on mould split lines etc - as we do not use spray equipment in order to reduce porosity of the gel layer. On a warm day these can be at 40 minute intervals, with a further 40 mins before the backup layer is applied.
This is then allowed to dry thoroughly - usually overnight - before applying 450 then 600 then the heavier bi-directionals. Allowing the back up layer to dry thoroughly reduces printthrough.
Hope this helps

 

I thought about brushing a small mold to see what kind of results I get. Anything I should know before I try it?

What type of brush is used?

 

You can get a gelcoat gauge from Fiberglass Coatings. Their website is: www.fgci.com.