This is my first time working with fiberglass. I'm working on a small fender for my race car. Ultimately, I'd like to do these and some other parts in carbon, but it seemed wise to spend some time practicing and learning with cheaper materials.
I made a mold of an existing fender using tooling gelcoat, fiberglass cloth and polyester resin. That worked really well, and I'm happy with the mold.
But, my first part did not turn out so well. In most places where there are relatively sharp bends, I'm getting large (1"-2" long) voids, which I assume are due to air bubbles forming where the 225g fiberglass cloth separates from the mold while curing.

Any suggestions on how to address this? Should I be using CSM instead of cloth? I've seen mention of others using spray adhesive to keep the glass in place, but I'm not sure how that might affect the mold release. (I'm using spray PVA.)

Original part (which was cleaned up after this shot, before working on the mold):


Top outside edge. The voids are clearly visible.



Leading edge with some clay and stuff still hanging on.

What process are you using to make the parts? It's definitely due to bridgining of the material in the corners, you need to ensure you force the material into the corners as a vac bag will.not give enough.

Thanks for the response.

As I said, I'm very new to this. But, basically:

1. Clean the mold
2. Spray PVA- thin layer followed by a thicker layer, let it set
3. Brush a layer of resin into the mold
4. Apply a layer of cloth. Roll it down, push it into corners with a corner rolling tool, get all the air out that I can
5. Apply another layer of resin, another layer of cloth, do three layers of cloth in total.

I'm sure  I'm doing many things wrong. I just don't know what they are. I'm not vacuum bagging, though I'm considering it and researching how that works.

Any insight would be appreciated.

It's not bad for not using a gelcoat. For wet laying fibreglass, you should apply a gelcoat in the same fashion that you did with the tooling gelcoat when you made the mould.

This won't inherently fix the issue, but it does somewhat fill in those sharper corners and make it easier for the fibres to stay in there.

To be honest, the corner problem will get much worse when you start using carbon fibre. Not so much because of the fibre, but because of the different characteristics of epoxy resin. This is why basically everyone uses at least vacuum compression when working with carbon. But in my opinion, if you use vacuum compression, you already have a pump and therefore could do a proper infusion instead. This will give better and much more consistent results. The clear gelcoat used with carbon fibre won't help you at all, because you will end of having the same imperfections under the gelcoat, wich is even worse. Gelcoat is great for cosmetically appealing carbon parts, but only if you got a void free process for making the laminate.

Some good advice above, but if you'd like to continue trying to improve results with your current wet-lay process using polyester resins, then applying a coat of clear polyester gel coat ahead of laminating your reinforcement will help

If you can get the timing right, then the tack of the gel coat just before getting to a more hardened state can help hold your fabric into the corners better. However, this doesn't really allow much scope for re-positioning things so you'll need to be quite good at laying the fabric in cleanly if a neat weave alignment is important. 

You might still suffer with some air entrapment, but hopefully not at the scale you've already seen and the gel coat surface on the finished part might allow for some repairs if any resultant bubbles can be pierce and filled

However, on parts with quite sharp corners and other detail, using vacuum is the best route forward in terms of achieving good cosmetic results, not least because it will help to force the fabric into the detailed areas and ensure you get a better fabric to resin ratio. Again, as stated above, if you do decide to invest in a pump, you could then move on to working with carbon and epoxy, with an excellent prospect of making very good looking parts after a bit of practice

Thanks to everyone for the help and advice. Looks like I'm off to order a pump and some fittings.