Painting mating surfaces

[Martin CooperParticipant @martincooper52042]

Hello everyone

I’m about 50% through building a 3″ scale traction engine, with all parts as yet unpainted.  Some parts are showing very mild surface rust and so I am starting to think some paint might help.  At this point, I realise I am a painting beginner and need advice please.

This screenshot shows the front axle, cradled in a spacer which then fits in the fork:

The axle+spacer assembly pivots around the pin and so there is some movement between the spacer and the fork.

Am I correct in thinking that the mating surfaces on fork and spacer should not be painted?  But if the non-mating parts of the fork are painted (ie, the bits you can see in the image) then fitting the spacer into the fork will most likely scrape off some paint from the fork.

Or perhaps the whole assembly is painted after assembly, with masking tape applied where appropriate.  But then the first time the spacer moves in the fork, it’s going to shear the paint.

Thank you in advance.

Martin

 

[Richard SimpsonParticipant @richardsimpson88330]

Running surfaces, be they sliding, rotating or oscillating are best not painted.  Such surfaces frequently work with minimal clearances for smooth running so paint can take up some of that valuable clearance.  The paint can either erode and mix with the oil to form a sludge that can lock up moving parts or it can come off in chunks and again seize up moving parts.  I also do not paint mating surfaces as the thickness of the paint can affect the fit and can make dismantling very difficult.  Mating surfaces are rubbed with steam oil before assembly.

Generally ferrous parts need painting to prevent them rusting however all running surfaces should be left bare.  It is far easier to paint the parts individually with the running surfaces masked.  I use an acid etching primer followed by coach enamel.  The running surfaces should then be kept oil free by frequent oiling.  It is also possible to blue the metal, which also helps to prevent rusting but it is not as effective as paint or regular oiling.

This refurbished 10V was painted as individual parts after masking all running faces and mating faces.  The crosshead guide was masked with a paper tube and mating surfaces were masked with masking tape.

A couple of coats of etching primer followed by a coat of two part epoxy paint:

All running gear is left bare with the ferrous parts relying on oil to keep them rust free:

Interestingly the cylinder cover was blued and simply needed cleaning up:

At the end of the day there will always be bare metal components that require attention to prevent rust forming.  I even polish cylinder covers and valve chest covers, which then need oil rubbing into them:

[Julie AnnParticipant @julieann]

On my 4″ scale engines, which do not have the blue spacer, the axle and fork are machined to be a rattling good fit, about 40 thou clearance. Both parts are fully painted, two coats of primer, two coats of high build undercoat and two coats of coach enamel.

On the full size engines I suspect that the axle and fork were not machined to a close fit, just left as fettled castings.

Julie

[Richard SimpsonParticipant @richardsimpson88330]

Interestingly another area where normal marine practise may vary from normal railway practise.

[Dave HalfordParticipant @davehalford22513]

Martin,

A thin coat of spray zinc rich primer (not the cheap five quid stuff, a good can is quite heavy compared to ordinary spray tins the same size) will stop the rust.

[duncan webster 1Participant @duncanwebster1]

Richard, photos not showing

[Richard SimpsonParticipant @richardsimpson88330]

Sorry Duncan but they are showing on my screen.  Can anyone else see them?

[duncan webster 1Participant @duncanwebster1]

not showing on PC or phone.

[Stuart Smith 5Participant @stuartsmith5]

Not showing on my ipad

[bernard towersParticipant @bernardtowers37738]

I notice that you use steam oil as a lube but that accepts water wouldnt you be better off using a lube that does not (hyd oil etc)

[Richard SimpsonParticipant @richardsimpson88330]

Maybe its file size, can you see this one?

[bernard towers]

On bernard towers Said:

I notice that you use steam oil as a lube but that accepts water wouldnt you be better off using a lube that does not (hyd oil etc)

I guess its open to discussion.  It absorbs water but it still lubricates internal surfaces, so you may be right.  I just use it because I have it handy.  Sometimes I use engine oil if that is lying around.

[BazyleParticipant @bazyle]

During a progressive long term build as a matter of course you should lightly paint all steel with clear Waxoyle diluted 50% with white spirit. This evaporates to give a slight wax layer that is pleasant to handle and protective. It will need proper cleaning before painting.
For rubbing surfaces like the above consider passivating the area with phosphoric acid or a cold black treatment. Both will wear off quickly in use but the very close areas not rubbing will remain protected. Both above, when de-oiled, can be painted without removing, a bit like an etch primer.

[duncan webster 1Participant @duncanwebster1]

Yes can see that

[Richard SimpsonParticipant @richardsimpson88330]

Thanks Duncan, don’t know what went wrong with the initial ones, I can still see them fine.  Maybe something to do with the fact that I use a rather large screen.

I can’t edit the original so I’ll repost the pictures here.

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