General Discussion (Model Railroader)

I recently picked up an old IHC/AHM/Mehano SD-40 (like the ones seen here http://ho-scaletrains.net/ahmhoscaletrainscollectorsresource/id15.html) that I would like to repaint in Chicago Great Western colors (Ironically, I purchased the C&NW #922 which was originally CGW #402)... I assume maybe some of you have repainted a few of these particular SD-40s before seeing as they were pretty fairly detailed for their time.

For those of you who aren't aware, the shell is basically in 2 pieces, if you look close at some of the photos you'll notice the handrails, frame, steps, etc. are black. I have been informed that this is because unlike the rest of the styrene body they may be made of delrin (which unpaintable according to many sources). Anyone know if this is true???

As far as I know, all the Mehano SD40s had a delrin frame. Etching or roughening the plastic is about the only thing I've seen around the internet that might work. Maybe you can coat it in a thin layer of clear plastic after painting to keep it from flaking and peeling?

BTW, some may tell you to toss the SD40 in the garbage and get an Athearn or something. I say, if it works well for you, and you like it, keep it!

Any suggestions on what generally works best for etching such slippery plastics???? I've been suggested regular vinegar, 91% isopryl alcohol, apple cider vinegar... Are there any better candidates out there??

 I'm not sure that you need to do anymore than clean the frame/handrails, as described. What you can do is check out the paints used to paint Lexan RC car bodies. The paint is flexible and will bend along with the frame/handrails when flexed. The paint is also specifically designed to hold to a slippery surface. You may have to mix different colors to match as well as possible, but the subtle differences can be fairly well hidden with weathering.

A tip I got years ago from a Weaver (O scale) technician at a train show was to prime the Delrin or any other acetal plastic with NAPA # 7223 flexible parts primer. It goes on clear and dries flat. You have to coat all sides. Then it can be painted with the paint of your choice. For the most durable results use Fascolor or Tamiya paint for R/C Lexan bodies over the primer. I have used Modelflex and Polyscale with good results as well. The techie demonstrated the effectiveness of the primer by tying the handrail from an O scale RS-3 in a knot and non of the paint flaked off or chipped.

I've been using this method for years on all of the models I paint and the paint just stays stuck.

Be sure to wash the handrails in alcohol and/or a degreasing detergent (like Dawn) to remove the mold release.

If you're using solvent based enamels, you can mix the primer with the paint. Autobody paint suppliers also sell similar products for mixing with paint..., but you have to buy it by the quart (about $35), the spray can from NAPA is about $8.

 Great suggestions from Modelmaker.  They do work as I've used several methods for adhesion promotion.

There is a "poor man's" option that can be applied on delrin truck side frames. 

With a toothbrush in good condition, pour a small amount of Comet or Ajax scouring powder into a small puddle of water in a dish (2 to 3 ounces).  Stir the mixture into a paste and scrub the truck side frames with it.  Rinse off, dry and check carefully for any slippery areas. Scrub again as needed, gettting into the nooks and crannies.  This should only take a few minutes. By the time you are finished, the side frames should appear dull and smooth with just very fine scratches.  Paint and weather as you would styrene .

OK, i feel the need to jump in here to clarify something a bit.

There is no  mold release agents used in injection molding processes. Now that I have made that very general statement I will concede that at times mold releases are used on occasion. This is very rare. If a mold release is used it is because they are trying to cover for another problem in the process. The most likely cause is a mold that has been run for too long and has developed a varnish like coating from the outgassing of the material. Any good injection mold shop will pull and clean the molds long before this happens as the damage caused will be very expensive to repair. The process used to run these parts is most likely fully automated. The machines today can run in full auto cycle mode where the parts are ejected onto a conveyor at the end of the cycle and the machine closes automaticaly to start the next shot. No time or provision to apply mold release.

Mold release compounds that are used in hart tooling molds (steel, aluminum, brass, Etc) are mostly a "paintable" release agent. The advent of these came about due to safety concerns of spraying a solvent based release onto a hot mold. some of these molds can be hot enough to ignite the older types of releases. The newer generations of release agents are safer in this regard.

Acetel (Delrin, Celcon  and about 600 other brand names) often refered to as engineering plastic is hard to paint and almost imposible to glue. It is used primarily due to its strength and flexibility. One other good trait, since it is hard to paint, is its ability to resist staining. The model manufacturers use it because of its strength. a comparible styrene handrail would not hold up to the handling that acetel does. The reason it is hard to paint has to do with its chemical composition. The explanation is long and to be honest with you confuses me as I am not a chemint. To make it simple though, it is not as porus as styrene nor will paint solvents 'attack" it to make a bond.

Sandblasting is one way I have heard to get paint to stick. The flexible primer is the best way though. I have used some of this here at work to paint prototype models with good results.

Although I am not a plastics engineer and do not claim to be an expert on this I do have 15 years experience with 28 mold machines and 6000 lbs of acetel injected per week.

Dan

Should I try roughening up the surface with some steel wool or the "comet trick" prior to an alcohol bath and flexible parts primer???? or will the steel wool not stand up to the strength of the derlin.

thanks for all the help guys...  I keep looking at those pictures of the "Bevbel" and Chessie System SD40s where there are multiple colors on the sill... And I know there has to be a way around this. I've never took on a task like this before, normally its Athearn right outta the box.

If you use the Napa primer, it wouldn't be a bad idea to wash with some detergent just get rid of any surface contamination (fingerprints etc), then just spray on the primer. Don't forget to prime all sides, as part of the strength comes from fully enveloping the item. You don't have to cover all sides with paint, just what shows.

I have two of these old SD40's that I received from my grandfather long ago.  I ended up stripping off the paint using Scalecoat II Paint Remover and it did not hurt the bodies at all.  I then painted them and they took the paint fine. They were my test units that I tried painting and decaling before I started in on much more pricy engines.  Here are my before and afters:

Do you know if the sill part on your models were delrin? Someone told me they may have only been delrin on the IHC releases, but I find that hard to believe seeing as they all were made in yugoslavia. Did you need to etch the plastic in anyway or use some kind of special (ie flexible parts) primer? Kinda off the topic, but how did you post those pics? I have been trying to figure out how to upload photos here for some time and it just doesn't seem to be working... after pressing insert photo button, it starts prompting me for a URL as if the photos have to already be on the web as opposed to uploading them. Lastly did you know that a bunch of MRL's SD40s originally belonged to the Chicago Great Western? Just thought it was kinda funny that you and I maybe trying to replicate the same locos just 35 years apart!

Sorry to get back to your questions so late but I'm now hardly on here anymore.  I don't know much about the delrin and yugoslavia.  I do know that I stripped the paint and put two coats of paint onto the shells.  I did not use any kind of primer.  I really never do on any of my paint jobs.

If you have not figured out the photo deal with this site then I'll tell you.  You need to go onto photobucket.com and set yourself up there for free.  Then take your photos and load them on the photobucket site.  Once they are loaded under the photo will have some differents things.  The bottom one is the one that you to for transporting them to this site.  Simply right click on the one on the bottom and then click copy.  Open up this page and click paste and there you go.  all done with a nice photo.