Whos good with fiberglass mold work?

[V]

I have a project i need to do, but it involves remaking a fiberglass part. I figure if i can set up the shape i need out of a solid form(like styrofoam, covered with clay) then i can just use fiberglass cloth and resin to make the new mold. It has to be pretty thick when done, about 5mm. I would like to do it with as little finishing work as possible since i dont want much body filler on top to smooth it out. how many layers would i need to do in order to achieve the thickness i want and is choice of which brand products i use a big deal?

[ins0mnia24]

What exactly are you trying to contruct out of fiberglass?
The reason i ask is because thickness can be achieved by using different ounce Fiberglass Mat. There is a variety of stitch mats and cross hair mats in different ounce thickness. As for applications of each mat it depends what you are using it for,where it is goin, How stronge you need it to be and how smooth you need it and if you going to be painting or gelcoating the fiberglass finish
If i knew what you were trying to make i could help you out alot in this department because i do happen work for a Company that builds Luxury Yachts.
Let me know and i'll tell you the easiest and best way to go about it

[V]

basically im reshaping a 3rd gen a/c-heater box. it just needs to withstand engine compartment heat. I probably will end up painting it, unless the final finish comes close enough to a stock look.

[ins0mnia24]

That would be a simple build the easiest way to do that would be
Go buy some Oaktag from Staples or where ever you can get it cheapest
Make your pattern and shapes with the Oaktag get it all cut out and tape it together so you have a pattern of the size of it and how it will look and see if it fits where it is being installed ect... Dou you happen to have a picture of what it should look like?? If so post it ..
The only trouble i see is getting 5mm Thickness that is really thin and structure wise Fiberglass resin by its self wouldnt be strong enough
What you could do is take your taped together Oaktag pattern then get some fiberglass mat and cut it out so you can apply it directly over top of the Oaktag this well act as a Form or Mold so you can retain all of the bends and shapes needed to build the box Once that dries you can take a 2 or 3 inch paintroller with fiberglass resin and roll it right over the Oaktag this will build a protective surface over top of it. then your pattern would be completely covered in Fiberglass
Normally i would use Carboard as a backing Mold or Form but since you need it so thin the Cardboard+fiberglass mat would probably be to thick so i would use Oaktag.
I know the Oaktag and Cardboard sounds iffy but trust me it works very well
I have used that technique on many things from Race seats,Dune buggy seats , Custom kicker boxes ect... Once the Fiberglass dries and cures 100% over both sides of your pattern it will be Very strong
And plus it is a fairly simple process just makes sure you have proper ventilation because the fumes are pretty bad and hazardous make sure you wear a mask.