I'm not a body/fender guy but I've played with glass and bondo enough to know it never acts quite like you want it to. My thinking on this particular repair is that if you do a good job pulling the dent the surface of the metal is going to come of fairly flat (uniform). Yes, you can put a layer of glass & resin over the pull hole, but when you sand everything down to the level of the rest of the metal, you will have sanded off nearly all of the glass cloth - thereby defeating your purpose and possibly even sanding right through to expose the hole again.
The best fix, obviously, is to weld up the hole and grind it down. The next best (IMHO) is to use the normal shade tree approach, which is to press the bondo into the hole allowing it to ooze through the whole and then expand a bit on the back side making somewhat of a "plug". Again, any fill of a hole of gap with bondo is NOT going to give you the best results. But if you are just doing a quick fix, it will last for a little while.
I don't have a welder and I'm concerned about the filler over the hole.
I read somwhere on here that no bondo is waterproof, I'm afraid
of moisture on the back side of a panel, like a door.
I have Dynaglass that claims to be waterproof but some say it really isn't.
Wouldn't fiberglass be more waterproof? I can dimple the holes in to
hold it.
Ducky if you are not going to get a welder... do this... get some short hair fiberglass... it has the fiber in it already... Fill your holes... sand down, cover with skim coat of glaze ( 2 part by the way, like Icing)..
Get access to the back of the panel and spray a rubber undercoating on it... Now you are sure you are water proof... I have used fiberglass before and never had any failures with it absorbing water, but there is always the first time for everything
Maybe I will be accused of heresy, but if you just can't or won't weld up the holes, sand the backside of the repair first, then fix the frontside beginning with something like Duraglass or Everglass. Once the repair is complete, return to the backside and sand it again, knocking down the 'worms' that have formed. Apply a coating that will prevent moisture from entering through the holes, primer and paint would be good, or even undercoating would be acceptable. Whatever you do, getting a good coating on the back of the repair is a critical element, no matter what repair procedure is used!
If you can't weld the holes a good fix is to use marine epoxy to fill or cover from the back side. It works with fiberglass matt (thin) or cloth and for the most part marine grade epoxy is water proof. Sand or grind and bondo sticks to it just fine. I get mine (POXY-PRO I think) at Menards. It seems to hold up better than fiberglass fillers as a moisture barrier and it's handy around the shop.
use 2 part body panel adhesive with a backing patch piece of metal on the inside of the hole (plastic filler and blend the patch if it's gotta be pretty)
use enough so it comes thru the hole a little, sand that down, plastic filler on top to blend your done.....
fiberglass cloth and resin is hydroscopic, gelcoat on fiberglas is the moisture barrier....
My take on this if you can't weld them would be to indent the holes a little then use something like panel adhesive to fill the holes first prior to plastic filler. This is if the area isn't low already, like trim holes. Being it is dented already the area may be a little low after pulling and you can apply it past just the hole. You can get small tubes adhesive from 3m I believe where a gun to apply isn't needed. You don't want it just adhering to the hole only. Fiberglass filler would be my second choice. The stud welders are a good investment, as well as a small welder. I've gotten plenty of use out of mine. I don't miss drilling holes for dent pullers or pull rods.
If you have access to the back side, why would you drilll holes to pull it anyway? You should be able to work out the dent. If you haven't drilled the holes yet, don't. Either buy, or find someone that has a stud gun and use it. If you are going to repair it, repair it, not just cover it up.
"like inside a door" (one of the worst areas for rust cause of wet and dirt and air circulation)...
take the door off, lay it flat, hang it up side down, whatever it takes to coat and treat the inside if you drilled holes or it will rust thru quickly from the inside
in other words....don't drill a door, pay a shop to stud pull it or hammer it out from the inside as Adcart wrote
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