[400TT]

Mick, just make sure you coat under the guards with some sort of body deadener, thick enough to block all light. Fibreglass is transparent and if you don't coat the underside of the guards, it will wash out your paint colour.

Using high-density foam/neopreme is a good idea to stop rocks etc hitting underneath and cracking the fibreglass and/or paint. Seen this on a few cars now, both front and rear guards. Not that common and I have never fitted the material myself, although these days I would.

After paint, just attach foam/neopreme. It only needs to be attached to the top surfaces(horizontal surfaces). Superformance use what looks like 4-6mm high-density foam just on the top surfaces, it's attached on top of the deadener. Neopreme is probably a better idea though, which is why Harrison themselves use it.

I wouldn't paint over the foam/neopreme. You want to make it easy to remove for body repairs if necessary.

Foam/neopreme also reduces noise from guard areas as it stops the echo of sand/small rocks(and some big rocks) being thrown up against the guards by the tyres. The stickier the tyres you use, the more this occurs.