Can you skip (most) sanding when using peel ply?

I was under the impression that when laying down several layers of a fabric like fiberglass, you had two options to ensure good adherence of the next layer to the previous: Lay down the next layer while the previous layer’s resin is still “green” to get a chemical bond, or let the previous layer fully cure, then sand before laying down the next layer.

Now I read on a wooden boats forum that if you use peel ply, the fine fabric weave imprint left on the epoxy from the peel ply provides enough grip for the next layer of fabric and thus does not require sanding, or at least requires only minimal sanding.

I’ve never heard of that. Is it true?

http://www.epoxycraft.com/asked-hamish-whats-great-peel-ply/

Answer is there. Says yes no sanding.

In actual fact, while the finish looks even, it’s actually textured. The fine weave of the fabric leaves behind tiny bumps and valleys in the cured epoxy and it’s these that your next layer of epoxy will adhere to.

Peelply leaves a matte textured finish. If you want a smooth surface you will need to make at least one more resin application to get it smooth.

If you can do so, when applying additional resin or another layer of a laminated patch, I would shoot for a chemical bond whenever possible. If time and conditions permit I will do so while the prior layer or application is still green. But with most epoxies, you can still probably get a chemical bond to some extent within a week of the prior application.

If you do need to let the epoxy fully cure, be sure to at least wash the surface with soapy water, then rinse well, to remove any possible amine blush that could interfere with the cure of a new epoxy application. There are epoxies that claim to be “no-blush” but I have not used them and can’t vouch for that claim. If you are making an application over epoxy that is still green, there is no need to wash.

To get a chemical bond, I tried peeling off the peel ply after about 3-4 hours but lots of gooey resin stuck to it and I pushed it back. It peeled off nicely after maybe 14 hours but of course that’s already beyond a “green” state.

Some resin will adhere to the peelply. Use of peelply does result in somewhat greater resin consumption. It is OK to remove it, so long as the underlying fabric does not start to lift off. But you will still get a chemical bond at 14 hours. Most epoxies take more than a week to fully cure.

when using Peel ply. Take a small piece of the same fabric you are using for on the project. When you wet out the project , also wet the small piece. {I like to put a small curl in the small piece} Put your Peel Ply on the project. monitor the cure by checking the small piece. When the small piece of fabric is somewhat cured , but still dent-able…you’ll know how cured the project is without trying to remove a gooey mess.

Best Wishes
Roy

@pblanc said:
Some resin will adhere to the peelply. Use of peelply does result in somewhat greater resin consumption. It is OK to remove it, so long as the underlying fabric does not start to lift off. But you will still get a chemical bond at 14 hours. Most epoxies take more than a week to fully cure.

Pete, how much time do you leave from one patch to the next? I’m working with G/flex at 70 degrees.

I have applied five or six layer patches in a single warm day, applying each patch one at a time. In that instance, I was not using peelply but applied the largest patch first, then progressively smaller and feathered and smoothed the entire repair as soon as the epoxy was cured sufficiently to allow sanding without clogging the paper. If you are using peelply, you can remove it and apply another patch as soon as the epoxy of the last layer is cured enough so that your material under the peelply does not lift off. The time required for this will depend primarily on ambient temperature, but also on humidity to some extent as well as what type and weight of fabric you are using. If you want to accelerate the cure rate somewhat, you can carefully periodically waft a heat gun over your patch to keep it warmer than ambient temperature, but avoid overheating the epoxy. The instructions that come with the G Flex will give you guidelines on how much it can be warmed.

I wish I had known about it when I built 2 S & G boats from plans.