dually fenders

[Donut]

Any tips, tricks for repairing these?  Both of mine are pretty smashed on rear of the fenders (smashed inward, PO said she backed into a tree with one, the other was that way when they bought it.)

I did start sanding one of them, they have had some work done.  Bondo in evidence, removed.  I did get SMC resin and fiberglass mat.  I'm attempting to do these on the truck, so they keep their shape, and the mounting bolts are a pain to get to and rusted.

Not looking for a show piece, these may be in epoxy primer for quite a while.  (try to seal them when I'm done with the 'glassing)

[Donut]

Well got one fender fairly back together, the other is stable.  Fixed spider cracks and the major ones, working on repairing a fist size hole.  1st coat on the hole.  Tomorrow I'll lay in a couple more layers and lay up some on the backside for further reinforcing.  (on all repairs)  I don't think it's too bad considering I've never done fiberglass before.

I'll finish at a later date.  (Finish shaping and paint,)  I was just trying to get it all stable and taking advantage of weather in the 60's

[Blazin]

Important to remember that the glass matting is the strength, not the resin.
Grind the area well with 24 grit. The scratches from the grinder ensure a good mechanical bond between new and old.
When grinding the damage go deep where the heavy damage is, and feather out a ways. Even a little beyond where youd think. Then rebuild the thickness with the mat, or cloth. Start with smaller pieces and work you way up to cover the entire ground area. When your done the heavy damaged area will be the thickest part of the new glass.
When laying glass use disposable brush to brush a coat of resin onto the area to be covered with mat. lay the mat onto the resin dry. Use the brush to dab at it until it has saturated the mat. You will get a foamy resin liquid. Wipe off with the brush most of it. You only need enough resin to just barley soak through the mat. Once the first layer is on you can do a few more but be careful not to move the under layer when doing this. Once you get the hang of it you can do 3 or 4 layers at a time. Then let it harden, rough shape it with an air grinder, then repeat. Once you have it filled up with glass you can run a block over it to high light the larger low areas. If there are any you can grind them and cut pieces to fit in them. Then fill them with the same process.
When filling a hole I use masking tape to make the shape needed. Once the glass is dry it will peel off for the most part.

[Donut]

Good to know that I'm on the right track. 
Good tip in the tape.  I've been using blocks of wood to kinda "form" it up. (tinfoil so the resin didn't stick to the wood)  Also using blocks and screws to stabilize bigger cracks until I can get them patched. 
After the outside is built up and solid, I'm also laying a layer or 2 on the inside of the fender  to span the area.
Maybe this little "tutorial" can help the next guy.

If I were still up north, this thing would have had a flatbed 20 years ago due to rust.