Removing old engine coating from IP2 housings

[vsb]

My first post so be gentle. I have an ex RAF Interpol which I’d like to civilianise without trying to make a classic replica as such. I have the engine in bits and with Graham’s help am sorted mechanically. The old coating on the engine was pretty rough so it has been chemically stripped and washed and has gone to my local vapour blaster. He called me over as he was surprised to find that when he blasted the residue from the housings they have a hard pale coppery metallic layer over the fins - in fact everywhere apart from the machined faces and the Nikasil chambers. Any suggestions as to what this is and how or if this can be removed or will I have to paint or powder coat to get an all silver engine?

[Richard Negus]

Hi,
Welcome to the forum.
Are you sure the surface you're seeing isn't the original silver powder coating that's gone hard and yellow with age?
Try scratching the side of an outside fin to see if that's so.
Usually, industrial strippers such as Thyssen Krupp in Tamworth get down to bare metal, no problem. Theirs is a hot dip process followed by high pressure jet washing.
R.

[vsb]

Pretty sure it isn’t paint/ powder coating and it has survived a prolonged dip in whatever our local alloy wheel refinished use + vapour blasting here in rural N Essex.
I wondered whether it was a left over from any base layer for the Nikasil plating process. Perhaps a more aggressive blasting process will remove it.
Great to get advice from Richard himself - it was partly the bike you built for Frank Westworth which lead to this project!
Vernon

[Richard Negus]

How about a photo?
Depending on the age of your IP2, the coating was probably done by BAJ Coatings (now defunct) at Weston super Mare. One of their chemical dips left a dark grey finish to the un-machined areas.
R.

[vsb]

IMG_5156.jpg

Not easy to photograph but note contrast between the machines face and the coppery fin. There is an unplated/ coated area in shot as well. Aviation thinners don’t touch it.

[Richard Negus]

"A picture's worth a thousand words....."

Fear not -that's normal. It's just discoloration by the pre-treatment and plating fluids. The joint face shows clean aluminium as it was ground to size after the plating process was completed.
All other machined faces are completed before plating and will show the same discoloration (inside the inlet port, for example).

I don't really know why that housing has a half-moon that's not discolored, but there must have been something there that protected the aluminium from the cleaning/plating fluids. A greasy thumbprint ? Perhaps Bob Rowley might have a definitive answer?
R.

[vsb]

So returning to the original problem: it looks like either more aggressive blast media to remove it or put up with a multicoloured engine if I wish to avoid paint or powder coating the housing?