HELP, Painting OLD Brass Locomotives

Clean, then paint.  I do it all the time.

Once in a while I do a light bead blast.  Lots cleaner, but the paint doesn't seem to fall off the cleaned tarnished brass, either.  Never use more than about 35 psi if you blast.

I usually use Scale Coat gloss black, but have been known to use rattle cans.  You need gloss fordecals.

I would let the patina grow, or polish. The heavy tarnish has to be removed anyhow to really paint it right. Ive never painted a brass train before, but I know most brass does not like thin spray paints, they flake off easy. If you must paint them, hit them with a coat of etching primer first. It will chemically etch the brass to help primer adhesion and prevent flaking of the topcoat. The etching primer in rattlecans of the better professional painter supply stores is usually kind of thin, follows/holds detail well, is even laying and hard as a rock in 3 days (dries in20min?). I wait for hard as a rock to neatly scuff sand with scotch brite before a topcoat. The etching primers I have used all had a unique green/grey look to them, a flat color, but so smooth it almost looks semi gloss. A shot of clear coat on the primer yields an interesting, industrial looking darker green/grey gloss color. 

since the brass is tarnshed, I think we can assume it was not clear coated. use a white vinegar solution to clean and etch the brass, then clear that off very well. use a good quality primer and apply a light, even coat. let that dry completely. you can even use an oven set at 200° for curing. once the paint smell is gone you can proceed with the top coat, again with light coats.

you didn't specify whether they are steam or diesel but I'll assume they need decaling after painting. Scalecoat paint gives a nice semi gloss finish that does not need additional glossing prior to decaling.

A fellow here recommended bar keepers friend.  I used it twice, it does an amazing job on brass.  I took the entire model and submerged it (except motor and gear box) in an aluminum tray with a full can of that stuff.  I scrubbed it with a tooth brush and the next day it was stunning.  It looked like shiny gold.  Air dry with an air compressor and then prime it then paint.  

I've done a few dozen brass engines and agree with Bob2. Just degrease and spray color. I use both Prep Sol and soap and water. Vinegar is probably a good idea but I have not tried that.  Then paint with Floquil. I preferred the old Dio Sol Floquil but the new formula sticks as well. it adheres with thin coats. Heavy coats just hide detail. 

This was painted over 40 years ago. No paint loss. BTW this started life as a '50s era MB Austin model.

Pete

get a rattle can of paint striper to strip the varnish off with.. then use metal prep wash with a red scotch bright pad..as you can see the train looks like new...then use mild dish wishing soap to wash off the metal prep..then use etch primer then prime it and after that paint it..all my trains are painted with automotive paints..last longer and more durable then those junk hobby paints like

 Floquil, Scalecoat.pics before paint and after paint..

Joseywales...can you tell me more about the automotive paint. For example I presume your bringing the paint store a sample right? How are they selling it, both quantity wise, and price wise. Does it take any special equipment to spray it beyond the usual airbrush gear most of us use? Your work looks quite good.

Thanks for the info in advance.

Bob

you can still use airbrush with those paints,just got to thin it alittle bit more..I use a small gravity feed HVLP touchup gun..heres other paint im going to try out..its the old Alkyd enamel..that automotive paint..I use urethane's paints on some of my stuff use that on my R/c race boats..it takes a good beating..its fuel proof..and smoke fuild wont hurt it..heres the link a guy sells lionels touchup alkyd enamel paints..good price too..my prr green on my S1 and S2 needed touch up..tried  Floquil, Scalecoat..both wouldnt match worth a darn..Took my tender to my local dupont paint store..they have a computer that takes pic of the surface..it will come up will a paint formula to match it..well they matched my 3rd rail PRR brunswick green to the T..couldnt tell were i did the touch up's..

http://www.ebay.com/itm/110849...id=p3984.m1423.l2649