One broken to failure piece, one not yet tested. Three of these were constructed using PC-7 epoxy (which is amazing stuff) to test what affect (or is that effect? I can never keep those two straight) surface preparation has on a joint quality. One set had no surface prep, one was sand blasted and another was roughed up using a fresh, sharp 36 grit belt.
Here you see the load test. I added 25lb bags of lead shot. Here is a picture at 50lbs where the metal is actually bending under the load and the joint is still holding strong. The size of this joint is maybe 1&1/2" square. I was really impressed with this PC-7 stuff. Maybe all the others would have held just as well. I may do a couple more tests to see but I was still impressed. All three held 75lbs, all three failed as I added another 25lb bag so I never was able to accurately measure, by weight, how much each surface held but we can still draw some valid conclusions I think.
The joint to the left is 36 grit, the middle sand blasted surfaces and to the right no surface prep at all. Starting from right note one piece ended up with most of the epoxy and the other piece had maybe 50% of the surface with epoxy residue. In all three cases, the epoxy failed, not the material, but in the test pair on the right, the adhesion partially failed by not sticking well to the metal. The left pair shows fairly even epoxy failure with 10% or so adhesion failure. The center test shows very even expoxy failure with no adhesion failure visible. The residue coverage was very even on both pieces. The conclusion we can draw is that sand blasting a surface is clearly the best method of surface preparation followed closely by a good, even ?scratched up? surface from a new, sharp 36 grit belt. I?ve been using a 36 grit belt but going forward I will be using the sand blaster to surface prep any where I can for better adhesion performance.