You're not saying what year car you have, but I'll just post a general answer that may help.
Ideally when you have rust and rust through, you ideally want to cut out the metal and replace it with new. However, if you don't have access to a welder or at least an acetylene torch to braze with, then you now have to use the substitutes.
I've heard of people using fibreglass and getting a pretty good result out of it, problem is, that if you're applying it from the outside, you will have a hard time blending it into the fender.
Another solution I've seen, is to use the POR-15 panel repair method. This stuff is basically a super thickened variation of super glue, which used with the cloth they have, works like fibreglass but uses a thinner patch and hence thinner repair overall.
They also have an epoxy that you can mix and apply from the back side and use that to fill the holes.
Ideally when you have rust and rust through, you ideally want to cut out the metal and replace it with new. However, if you don't have access to a welder or at least an acetylene torch to braze with, then you now have to use the substitutes.
I've heard of people using fibreglass and getting a pretty good result out of it, problem is, that if you're applying it from the outside, you will have a hard time blending it into the fender.
Another solution I've seen, is to use the POR-15 panel repair method. This stuff is basically a super thickened variation of super glue, which used with the cloth they have, works like fibreglass but uses a thinner patch and hence thinner repair overall.
They also have an epoxy that you can mix and apply from the back side and use that to fill the holes.