Peel-ply vs plastic sheet

Posted by Laszlo on Mar 28, 2005

Hi guys,

There's 2 types of plastic that get used a lot during glassing. Peel ply is a woven cloth that is used during vacuum bagging, as Scot says, and for the interior layers of a lamination. Smooth polyethylene (such as 4 mil dropcloth) is used for the final outer layer.

Peel ply, since it's woven, conforms more to compound curves. It's also porous and will wick up epoxy. Polyethylene is stiffer, will wrinkle on compound curves and does not absorb epoxy.

If you put peel ply on top of glass and press it in with a squeegee, the excess epoxy will come through the pores and sit on top of the peel ply. After the epoxy cures and you pull the ply off, the result is a piece of glass with the perfect amount of epoxy and a matte surface with the weave plainly visible. This is exactly what you want on the inner layers of a lamination - no excess weight, no floating glass and a surface with some tooth for the next layer to grip.

If you do the same with a polyethelene sheet, the excess epoxy is trapped under the sheet and forced back into the glass. This time when you pull the sheet off, the weave is completely filled and the surface is as smooth and glossy as glass. If you did this with an inner lamination, the layup would be too heavy and there is a good chance of having the glass float. But on an outer layer, with right amount of epoxy, it can eliminate almost all the weave filling and sanding work. Almost, since the sheet does not conform to compound curves, there's a good chance of wrinkles. Easiest way around that is to use many smaller pieces, but then there's seam lines to take care of.

Finally, somewhere along the way the people started calling the sheet "poor-man's peel-ply". It's nothing of the sort, as my explanation makes clear, I hope. It can be used as a poor man's vacuum bag because when squeegeed into the epoxy, the atmospheric pressure and surface tension will hold it in place and allow it to hold glass in place in situations where it normally would move, as would vacuum-bagging. It can also force epoxy into areas where it normally wouldn't go and hold it there.

Hope this helps clear up the plastic saga.


PS - the polyetheylene sheet can also be used to keep the workbench clean. :-)

In Response to: Re: Bulkheads In or Out? by Scot on Mar 26, 2005