Hi there, i'm looking to form a plug for a new painted engine cover. I'm proposing to use cross section templates taken from the attached cad file, copied and cut out of 50mm celotex or kingspan insulation. I have used this method previously and once sanded to shape, covered it in car body filler. This was successful, however the skin formed over the foam plug was brittle. I'd like to be able to use a spray or paint finish that i can apply directly to the foam and flat down, or i need to skin the foam with a layer of glass fibre prior to building up coats of primer filler? If i skin the foam with glass fibre, which materials do i need to use and how much depth of construction should i allow for when cutting the foam section in order to ensure my finished plug will end up the desired size. Any advice or direction to any previous posts that cover this topic would be really useful. (The green area, as shown on the attached, I'm proposing to form out of the foam, with the pink constructed in MDF, is this a good idea or should i do it all in the foam ?). This is all for a one off item.

This video tutorial should help

https://www.easycomposites.co.uk/learning/how-to-make-an-accurate-composite-pattern-by-hand

Thanks for that its really useful. Do you know if i would be ok to use the same procedure using kingspan or celotex, which it think has a density of only 30kg/m3?

yes you can (and I have a number of times) celotex tends to ball and grab when sanding, it's reasonably stable if supported correctly and i believe it is unreactive to most resins (although I have only used epoxy on it myself so best to check)

I've always glassed the plug then sanded & finished, you just have to make the foam part a little smaller to allow for the glass thickness

I used a polyurethane pour in place system that was only 30kg/m3, so I think it will work fine. I also used 3mm balsa wood as the skeleton structure. Would have been easier to use pvc foam as the balsa is quite hard to cut, but I could only get 6mm minimum here. Balsa is also much cheaper. 

Thanks for the advice, and the video is great. Can anyone tell me what i need to lay over the foam once shaped, prior to finishing. i.e epoxy matting or fibreglass tissue. Ideally i want to keep it as close to the shape formed in the foam/insulation board. Is there any thing i can stay or roller on prior to filling and building up with high build primer to keep as close to the foam shape as possible? Thanks, Simon

I used microballons in epoxy. Works well, make it as thick as you need, then use body filler for the bigger gaps.