Here is the back tips of the quarter
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That large of void You should trim away all the diseased metal back to good metal, square the hole up, acid etch everything & weld in patches. Keep in mind that welding heat is going to cause distortion in surrounding unstressed sheet metal, creating extra surfacing bodywork. You can trim & clean the area well, etch with acid(inside too)& stuff it up w/ epoxy filler thereby avoiding extra bodywork due to heat distortion & the car will probably rot around the repair before it gives up. Also save considerable time opposed to patching with steel, but hardcores think it an improper way of doing things.
aeromarineproducts.com
in San Diego carry epoxy & glass components. To make epoxy filler:
#1, the 300/21 epoxy resin(300) & catalyst(21)
#2, 1/32" milled fibers which is uniform ground fiberglass
#3, aerosil or cab-o-sil fumed silica which is actual glass..the silica is passed over high heat flame & the droplets are collected to make product....it is very light & offers extra structural integrity to mix...it is also a stiffener for the mix...one can make mix stiff enough to apply upside down & it will not move while the kicking process is going on.
By volume i usually mix the 3 components in equal portions.
The epoxy is 2 to 1, 2 parts resin to 1 part hardener,, ratio is critical & NOT to be deviated from. So if one mixes up four oz. of epoxy by volume, then four oz by vol. of milled fibers & four oz. by vol. of aerosil. This is the ratio for the initial coarse fills. As one progresses towards finish coats, the ratio of milled fibers & aerosil can be tapered off, making smoother mixes to fill smaller flaws.
The epoxy requires generous cure time, like one should wait a minimum of 24 hours before sanding. There is very little shrinkage w/ 300/21 epoxy. A single application should not exceed 1/4" to avoid 'resin boiling.'
However, once the spread has gone off, which is about an hour in 80 to 90 degrees, further application can be administered without any prior sanding, so long as 1st application hasn't been contaminated by airborne impurities such as oil, silicone, ARMOR-ALL, wax etc.
I know i'm pounding a lot of info @ You. Aero Marine has lots of instructional, technical info & product line vids on Youtube. Also They are quite congenial on telephone, willing to take time & explain.
Bondoe is nothing more than polyester resin w/ talcum...it is NOT a structural agent! Epoxy is 6 times stronger than polyester & TOTALLY water proof, polyester is NOT, it will absorb moisture under prolonged exposure. Polyester shrinks @ least 10% of its volume, epoxy less than 0.5%.
*also, polyester resin can get very hot during kick process, enough to cause heat distortion in lightly stressed panels(i've done it more than once) epoxy makes about a tenth of the heat during kick.
I made a silver dollar sized puck, about a 1/4" thick of the epoxy filler in the component ratio described above. After 24 hours i put it in hydraulic press & dialed in 12 tons of pressure...no fractures. A week later i put in press & took it to 18 tons, couldn't exert anymore due to hyd fluid leak. 18 tons Bro!! Only at extreme edges of puck did i get tiny light fractures, the puck compressed 10 thousandths.
Some of those floor pans of Yours that are only lightly perforated, You could grind, etch both sides, & spread epoxy filler(one side at a time). Back one side with masking tape, spread on the filler, let it kick, then go to other side, pull the tape, wipe down with acetone & spread that side with the epoxy. You'd save Yourself a ton of time.
No matter how stiff You make the mix, You can spread it as thin as You want. You have to develop a feel for the spread, it's not like spreading bondoe.
*important note* with ANY filler, first spread thin with Force the entire area, so it's well pushed onto surface, then You can lighten Your pressure while spreading to achieve a build up of material.
I started a thread on here back in '14 called
Filler secrets 101
Remember...18 TONS!!!!!!!
aeromarineproducts.com
