new to forum, couple questions about my fiberglass box...

[lanceam24]

I just finished a fiberglass sub box for my 2001 Mazda MP3 (coincidence?) Everything went smooth; the structure is solid, sounds nice, fits like a glove. I have everything i need to paint it and I would like a piano type finish to it. I have sanded it down and primed a couple spots. For the most part, it looks lumpy? or wavy. I have applied some body filler to certain areas, but i think i did it wrong (applied filler, dried, sanded it all off until only little holes r filled, plus i used an orbital sander) What is the correct method of applying body filler to smooth it out and get rid of the waves and lumps?

[thekl0wn]

You will need to use a wider spreader, and use it to even out the surface... Where you have high "lumps", the filler should barely cover, whereas the low points will have more filler in them to level them up with the existing high points. Just make sure you're using a decent filler... Possibly a fiberous filler.

[djvillar]

Remove Thick Primer:
Sand everything down to remove that primer, you want your filler to bond to the structure not the paint. For this use 30 or 40 grit so that your surface has "teeth" and hand sand cause you want to get the paint out of the low spots.

Guide Coat and Block Sand:
Now, I know you would have just removed the paint but... put an even but lite coat of paint on there, a color that'll stand out. Now block sand again with low grit paper, the block should be completely rigid. You want to use a long block not an orbital dual action. And orbital will give you feathered edges and get into the low spots.

Filler:
definitely use a wide spreader. If your not extremely comfortable with working with filler, mix small amounts and focus on one 6x6 or 8x8 portion of the box. Once it starts to cure trying to spread it is pointless. Do small portion so that you can mix another small, fresh batch to cover another 6x6 or so portion while the first spread hardens up a bit.

Tip: when spreading apply force to the outside ends of your spreader (not the center) so as to "bridge" the high spots and fill evenly.

Sanding:
If your surface is complex I like to use a malleable block to sand (not an orbital) again with low grit. With low grit if you hand sand you'll unevenly apply pressure and create low spots, with an orbital the surface might look be very nice and smooth but have low spots where your edge may have lingered. If you use a thick sponge or a flexible piece of plastic as a block and low grit paper your sanding block conforms to the overall curves like your spreader did. Low grit paper will help "shape" the surface more quickly and don't mind that you're going to be shedding quite a bit on filler... it "filler" not structure, you don't want tons of filler in the end...

Tip: if using a sponge or flexible plastic as your block, take your sand paper spray some adhesive on the back and fold in half. This way you have two sided sand paper, one to sand and the other holds the block without slipping around. When it gets loaded with filler flip over and con't.

Finish Up with Glaz:
After all that sanding with low grit paper, your surface should be uniformly contoured but devoured with course scratches. No worries. Get some glazing finish putty. spread it very thinly all over those scratches. Let it fully cure, place under some lamps to speed the process. Now hand sand with 500 to 1000 grit, Hand sand cause with this kind of grit your not going to harm the filler at all. Spray some Primer/filler, let cure, and hand sand again with that high grit.

Your surface will feel like glass!!!

[djvillar]

Take more photos as you go. Keep us updated. Good Luck.

[lanceam24]

ok, followed ur directions to a T. everything turned out fine... until i started spraying on the paint. the spray gun sputtered in a few spots and i have a couple large runs. how would i go about getting them out? sand with a high grit sandpaper when it dries? i am about 95% of the way. as soon as i figure out how to fix the runs, i can apply my clear coat and u get pics

[djvillar]

Depending on how thick those runs are when it cures up you might need to sand with something with some teeth to it, 120 or 150. Whatever get it back to level. Glad to hear its working out.

[lanceam24]

so just sand the runs down and paint over the top again?

[djvillar]

Yup. If they are think, you might need to use that 120 to knock it down, in which case you'll have to sand in stages again. 120, 240, 360, 500... until its silky smooth again before you paint.

[lanceam24]

overall i am happy with the result.
couple things i did notice when painting tho:
-get quality sprayer and compressor
-dont paint outside in louisiana lol

[djvillar]

Dude!.. I would never had known you had any issues with filler at all. Well done.

[lanceam24]

glad u like it, thanx for help and support