I started the repairs of stress cracks and gouges this weekend. I sanded them out (sometimes all the way through) with 80grit wrapped around a Coke can, then filled them in with short strand fiberglass filler. It was tough to get shaped right (mainly because I'm inexperienced, I think) But I managed to get it close. I waited for it to get stiff, then grated it down and sanded w/ 80grit on a block to shape. It left pinholes and holes larger than pinholes, as you can see:

Does this look alright? I'm planning on putting a layer of Plasticworks on top of this for final filling.

 

Also, is there a grinding wheel of some kind that's safe to use on fiberglass panels? (Won't leave paint-unfriendly particles?) Some of the major stuff takes a zillion years to grind out with 80grit.

 

Would use 35 to 40 grit to shape your patches or fill work first. Then go to finer grits for finishing up. Can use angle grinder with flex pad on it for sanding. Be carefull about how much pressure you put on pad so you dont leave flat spots or gouges in your body. Remove paint, primer and firstshaping of body repair with this. Body supply shops sell sanding discs with self adhesive on the back of them to stick to pad. When finish sanding would use blocks, long or hand for flat panel finish. Take you time, with a little practice you will become skilled at fiberglass work. Good books for C3 body repair is available at
www.glas-ra.com. Have books for all corvettes except C6 I believe, houstonvett

 

Just add more filler to them holes Boy!
When you are done with the short strand it should be almost perfect.

Then there is a finish filler that is quite runny that you use to fill the pin holes. Then a high build primer to fill scratches etc

 

Cool. I'll use more short strand to fill up the holes again (rather than using Plasticworks, right?). Is there a better application technique I could use to make sure I don't get these big holes in my patch?

And is there a small (Dremel sized) grinding tool I can use to carve out cracks, etc?

 

One thing I found is a "must have" when doing my car was sanding sponges. C3s are so swoopy and full of radius' that sanding can be a challenge, but the sanding sponge helps out TREMENDOUSLY! Got the tip from the book "How to Restore and Modify your Corvette 1968-1982", and I swear by them.

I used the ones you can get at a hardware store or harbor freight store. They are actually used by woodworking people to sand in crevices like on an old fashioned wood entry door or cabinet door, where you have recessed panels and such. Anyhow, the sponges come in various grits and are firmer than a regular washing sponge (dont recommend one of those. too soft). The sanding sponges can also double as a backer even when wore out. I just bought the 150 grit sponges to get the fiberglass work close and then wrapped a piece of 320 around it for finishing up. You can hold the sponge at an angle and use the corner too, for getting into the contour better.

I swear by these after using them. You can push on them firmly and they will shape to the parent contour. Similar to your Coke can, only more versatile.

When I bought my car, it had a cracked lip from a front tire blowout. I had to rebuild about 3 inches of nearly the same area of wheel lip you have in your pic. It looks perfect now. The sponge really helped. Particularly with that sharp radius there.

Hope this helps. Good luck.

 

Use resin and mat to do the repairs. You have far more control and its cheaper with a stronger fix. Its not like slopping old pudding on but layering on. No pinholes or gop piled too high.

Just paint some resin on and lay some mat on. Saturate and keep repeating. The mat should already be cut into about the size strips you want. dabbing the mat with resin usually is better than painting it. It gets it wet without the mess.

If your set on using the glop out of the can then cover it with some thick plastic sheet or wax paper. Now you can smooth it down. a trick in some areas.

I also like gelcoat for doing the final finish. Its easy to apply and easy to sand. Doesn't cure in like 5 minutes like bondo does so you have time and less waste.

After any of the 'finished' repairs wipe the area with water or solvent. The idea is to see the shiny surface. You can also wipe your hand across a surface to feel what can't be seen. Both methods will show whether you have a paintable surface or whether it will show faults.


There is also a "guide coat" method. When your repair is complete and ready for the final finish or layer. You spray a mist of opposite color and sand this. If white on black the white will show any low spots easily. Use laquer not enamal. Enamal clogs the paper too easily. This would be the final surface ready for paint.

If your getting it painted don't use any fillers of any kind bondo , gelcoat, whatever. Fiberglass is fine but a professional painter is going to remove fillers and do it themself. This base preperation is their painting reputation. They'll see any odd spots as old and will want to replace it with new. They don't want their paint job popping off or showing defects.

 

It looks like you are trying to pamper the fiberglass resein and letting it do what it wants...which is to say that those holes are air bubbles. You have to use some force to press the fiberglass into the shape that you want...put some drop cloths underneath to catch the drips...if it isn't dripping off the fender then you are waiting too long to start applying. It doesn't really matter how neat the job is of putting the resein on...don't sling it around of course, but built it up real good...1/8" taller than necessary if this is your first time forming a shape. The fiberglass comes off real quick with a sanding disk. Do yourself a favor though and get a small vibration sander...Craftsman makes one for like $20 that I used on my bumpers. 120grit for just 15 seconds can take off 1/8" and leave it relatively smooth.

So...in summary:

1. Application goal is to avoid air pockets...pack it into the holes and spare no expense.

2. vibration sander with 120 grit to start your shape. You may still get pin holes, but no large holes like you have...but even then just fill them in real good and keep building and sanding with 120 until you have a nice smooth surface.

3. vibration sander with 360 grit to take care of details on the shape.

4. You then want to go to a wet sandpaper of like 600 grit and hand finish the shape.

5. Now that you have the rough shape completed you need to get the finish smooth as glass with 1000 grit or 1500 grit depending on your skill. (2000 is too fine at this stage...it's only for final sanding on the clear coat before rubbing compound and wax/Zaino)

6. Prime the area with several even coats...experts can get away with no sanding in between. Inspect the job and make sure it's still smooth in between coats.

I also second the posts on the sanding sponges and guide-coat methods...you need some really good lighting to see what you are doing...a dark garage just won't be good enough.