Repairing and prepping for paint a damaged primary. Any advice?

I have been working through my pile of parts for my restoration and I realized my dirty primary case cover had a crack in it! It looks like it was laid over on the left side and the footpeg mount was pushed into the case. Only a slight dent and hairline crack, but bad enough that sanding and polishing would never hide it. I am thinking of using low temp. aluminum repair rod on the crack from inside the case backed up be an epoxy patch. Outside I will rough up the aluminum and spray with epoxy primer, lightly fill with a dab of bondo, then sand filler and primer with 400G, and finally paint with a catalyzed gloss black urethane. Will this work or am I asking for trouble. Any thoughts on prepping the aluminum beyond sanding and thorough cleaning? After looking around on Pinterest I found a few bikes with (gloss black) painted primaries and even one with a painted trans. cover and it actually looked really good.

That is a big piece of aluminium and I doubt you can get it up to the temp for the low temp aluminium repair rod just with a propane torch. I would do what I did and take it to a good aluminium welder, Brit alloy welds better than Japanese alloy. Mine was welded front and back, left the back as welded and dressed the front until flat and polished the whole outer cover.

From my Experience. First off JB weld will do the repair on the inside make the coat about 1/8"+ and cure overnight, It would also be a good filler for the outside "body work." NO welding needed. V out the crack and roughen the surface so the JB weld has a good surface to cling to.

BUT, the aluminum covers can be very porous, and you need to Boil or Bake as much oil out as you can with heat, once you start to heat the part you will see it oozing out of the pores, and then using products like Purple stuff to clean and remove oil film when you get the oil out after several heat cycles. Since you are going to paint, you are not worried about the dull surface it will leave. I think it will actually etch the surface which lots of paint finishes want done before priming. There are spray can products that etch and prime bare metals.

But if you have oil in the pores most paint surface or attempts to aluminum weld , will not like having oil bubbling up. From my Experience, take it for what it is worth.

@Carl H is correct, paint doesn't stick to aluminum very well due to the aluminum oxide layer on all unfinished aluminum. Paint will just flake off pretty quickly. For paint to stick, you need to use what's called a "conversion" coating before the paint, such as alodine (the greenish-yellow finish you see on military aluminum parts). It's expensive to have it done and it won't look very good.

Low temp aluminum rod will never stick either, as @Carl H said, due to the oil, and it basically sucks anyway.

The way to do it is:
(1) drill a ~0.060" - 0.100" hole at each end of the crack. If you skip this, you run the risk of the crack just creeping along in the future, from vibration if nothing else.
(2) grind a v-groove along the crack on both sides. This will remove the oil along the weld bead.
(3) Clean thoroughly in hot carburetor cleaner. Don't use any sort of caustic cleaner and do not use any sort of wire brush along the crack. A wire brush (steel or brass) will transfer metal bits to the aluminum and make it impossible to get a clean weld.
(4) TIG weld with filler rod on both sides along the groove.
(5) Grind the weld bead flush with the surface on both sides and re-polish the whole part. The weld may still be slightly visible, depending on the filler rod the welder uses.

I understand the crack is primarily cosmetic, but there just isn't a way to patch it very successfully without going through the correct process. I've been down this road enough times.....

Carl H said:

From my Experience. First off JB weld will do the repair on the inside make the coat about 1/8"+ and cure overnight, It would also be a good filler for the outside "body work." NO welding needed. V out the crack and roughen the surface so the JB weld has a good surface to cling to.

BUT, the aluminum covers can be very porous, and you need to Boil or Bake as much oil out as you can with heat, once you start to heat the part you will see it oozing out of the pores, and then using products like Purple stuff to clean and remove oil film when you get the oil out after several heat cycles. Since you are going to paint, you are not worried about the dull surface it will leave. I think it will actually etch the surface which lots of paint finishes want done before priming. There are spray can products that etch and prime bare metals.

But if you have oil in the pores most paint surface or attempts to aluminum weld , will not like having oil bubbling up. From my Experience, take it for what it is worth.

Click to expand...

Kommando,
Thanks for the feedback. I am sure you are right about a heliarc weld being the best repair but I live in an extreamly rural location and the closest welder that may agree to do a job like that is a 2 1/2 hour drive away so I will try something easier, more manageable, quicker, cheaper, and yes, probably less reliable. The paint I am using to prime is a 2-part epoxy primer I have used on aluminum and roughed up chrome with excellent results so I think it will work in this application if I can get the case very clean. I needs to be sanded smooth before base coats spray but the adhesion is as good as any paint over raw steel.

Carl,
Thanks for the good advice. I have followed your suggestion and I am feeling confident it will be an adequate repair.
I did heat the aluminum to very hot to see if there was any entrained oil. I did get some ooze in a few spots around seals but nothing at the repair site. Also selectively ground down to clean metal and roughed up a lot with a carbide scribe. The aluminum did not have any visible porosity at all, actually a very good looking and consistent casting so maybe got lucky in that regard. The Z-Plates on the other hand have some very obvious casting flaws which I think will be impossible to hide so plan on priming an painting the recessed areas and only polishing the edges and perimeter area.

I'd be on eBay looking for a good one to save the hassle. But that depends on where you are in the world, shipping cost can hurt. Me, I'd JB weld it haha.

I think the process that Carl H recommended is a good way to start and I have had good results using JB Weld. I also think your using 2X paint will work well along with the 2X primer. I painted various parts with these spray cans (SprayMax) and am pleased.Just remember you have a time limit for using these products.
Mike

I've JB-welded quite a few aluminum bits, often scraped-up engine covers, like these on my Suzukis. No problem with paint adhering (VHT)

tinkertoy said:

I have been working through my pile of parts for my restoration and I realized my dirty primary case cover had a crack in it! It looks like it was laid over on the left side and the footpeg mount was pushed into the case. Only a slight dent and hairline crack, but bad enough that sanding and polishing would never hide it. I am thinking of using low temp. aluminum repair rod on the crack from inside the case backed up be an epoxy patch. Outside I will rough up the aluminum and spray with epoxy primer, lightly fill with a dab of bondo, then sand filler and primer with 400G, and finally paint with a catalyzed gloss black urethane. Will this work or am I asking for trouble. Any thoughts on prepping the aluminum beyond sanding and thorough cleaning? After looking around on Pinterest I found a few bikes with (gloss black) painted primaries and even one with a painted trans. cover and it actually looked really good.

Click to expand...

Personally, unless you are going for concourse, I would just V groove the crack on the inside, and apply a bead of quality silicone sealer (e.g. Threebond) and leave it at that.