Prep & Paint for Old Pitted Mag Engine Covers?

[mcman56]

I have a 76 TL250 with pretty severe pitting on the outside of both magnesium engine side covers.  They are structurally sound and this is not a restoration so I'm not concerned with a cosmetic repair. However, I would like to prevent further corrosion.  For better or worse, they are both now bead blasted clean.  This is an air cooled bike but must have spent some time parked near water.  I'm in a mostly dry climate so there should be no significant moisture exposure.     

• Some on line sources say that Alodine or some other "conversion" prep is needed first to prevent corrosion.  Other sources say that Alodine will actually promote corrosion in magnesium and make it so paint will not adhere well.  These chemicals do not seem readily available or cost effective plus can be hazardous so are not desirable.  I do think I saw some gold tint on a cover when blasting but if it was Alodine, it did not help the original finish.   
• All sources seem to agree on a self etch primer but it looks like these are mostly lacquer based except maybe the Rustoleum brand.  They also come in different colors like green and gray which must signify some difference.  Is one better than another for this purpose?  
• I plan to top coat with some enamel roughly matching the original Honda paint color. 

Does anyone have experience with something like this?

 

Edited November 28, 2021 by mcman56
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[technowaldo]

Just prime it with a cellulose etch primer and top coat it with 2k 

[suzuki250]

12 hours ago, technowaldo said:

Just prime it with a cellulose etch primer and top coat it with 2k 

 

Etch primer and a good top coat, acrylic or High temp paint? 

(I’d avoid 2k as its not good for your health, but it does give an good finish)

[b40rt]

Bear in mind some / all this corrosion may be due to your engine acting as a sacrificial anode, so it is equally important to make sure the frame is rust free and painted.

[mcman56]

5 hours ago, b40rt said:

Bear in mind some / all this corrosion may be due to your engine acting as a sacrificial anode, so it is equally important to make sure the frame is rust free and painted.

This is an interesting comment.  I also noticed some odd looking rust on the bottom frame rails that were probably under the skid plate.  The rust is protruding up from the surface, sort of like crystals growing on the surface.  There is not much but it does look odd.  I was planning to remove rust and paint this area.  Other areas of the frame just look a little rusted and similar to what is found on other bikes this age.  Are you suggesting that the frame should be totally rust free?  The skid plate is also steel and will get cleaned and painted.        

[b40rt]

Yes, as much is possible👍

[mcman56]

On 11/29/2021 at 2:21 AM, b40rt said:

Bear in mind some / all this corrosion may be due to your engine acting as a sacrificial anode, so it is equally important to make sure the frame is rust free and painted.

This is an interesting comment.  I also noticed some odd looking rust on the bottom frame rails that were probably under the skid plate.  The rust is protruding up from the surface, sort of like crystals growing on the surface.  There is not much but it does look odd.  I was planning to remove rust and paint this area.  Other areas of the frame just look a little rusted and similar to what is found on other bikes this age.  Are you suggesting that the frame should be totally rust free?  The skid plate is also steel and will get cleaned and painted.        

[mcman56]

Looking at self etching primers, none show as heat resistant to go with engine paints.  On top of that, engine paints seem desirable due to resistance to fuel.  Are there any specific recommendations?