how to make fibreglass box smooth?

what is the stuff that people use to paint over the fibreglass sub box so that they can sand it down to make it smooth? Is it a 50/50 mixture of body filler and resin?

Well you just pour the resin..let it cure....bondo....sand...bondo...sand...then when you have it smooth enough...wet sad. You also want to go up in the grids when you are sanding..~The Rookie~

Rookie of the year that is...

Don't let the smoke out of your equiptment..it doesn't go back in.

Also if you are painting it...you will know when its time to prime it to paint because..you won't see deep scratches in it. ~The Rookie~

Rookie of the year that is...

Don't let the smoke out of your equiptment..it doesn't go back in.

i normally just use bodyfiller and a very low grit sand paper then work my way up in grits till i get to wet sandingdream it, build it, fiberglass it

once you stretch your fleece or trunk liner and resin it, sand it with 50 or 80 grit, preferably on a dual action sander to speed it up. once that is set, strengthen it from the inside with chop mat and/or other methods. then if there are any large waves, dips or imperfections, fill those using a reinforced bodyfiller first, if they are large. if not just use a standard body filler. sand the reinforced filler with 24 grit. if you catch it right as it starts to harden you can save yourself a lot of head ache sanding. it will just clog the paper fast, same with body filler. once you have all the imperfections filled and sanded with 36 or 80 grit. spray on a good 3 thick layers of polyester primer. this will fill ALL the sanding scratches and slight imperfections. once that is dry( 2 hours) sand it with 80, 220, then 400. if there are ant pin holes fill those with a polyester finishing putty and sand them to blend. once all that is set, sanded, and smooth, spray it with a surface primer(NOT rattle can) then block sand with 320 or 400 then to 600 wet. wipe it clean with wax and grease remover, the base, then clear.

Very helpful, thanks audiomechanic

I read somewhere that somebody thin-out the bondo with alcohol...makes a smoother (where's the spellchecker!) finish when dry and cuts down on sanding.  Don't remember where i read that though.

they make a spray that you can put right on it, its basically a spray polyester.  kinda like body filler but applies faster and smoother and its really high build, you should put it on after you sand down the fleece with 80 grit.  should take up most of the gaps

stuff i have is U-POL(TM) Reface.  its a polyester spray filler.  i got it at a local store auto body supply store thats close to my house.  i'm guessing a local place by you will have it or something like it