OK, I can understand now that car cement is somewhat like car undercoating, a petroleum based product- . If so, then I would suspect that it would be thick (i.e. have some bulk) and would be expected to at least "round off" or obscure the finer details (I picture the coating to be not unlike very thin asphalt paving!). However, in photos it looks to have a glossy "skin" when newly applied, apparently enough that it can accept painted numbers, etc. However, it would seem probable that dulling of the finish would occur fairly quickly, and that in our modeling a flat black finish would be most appropriate, even for cars that are relatively new.

Do I understand this correctly?

Denny
--
Denny S. Anspach, MD
Sacramento

Denny;
Yes, that slick finish did not last long. It was a sticky substance,
and therefore, a coat of dirt would adhere rather quickly.

Elden

--- In STMFC@..., Denny Anspach <danspach@m...> asked:

OK, I can understand now that car cement is somewhat like car
undercoating, a petroleum based product- . If so, then I would
suspect that it would be thick (i.e. have some bulk) and would be
expected to at least "round off" or obscure the finer details (I
picture the coating to be not unlike very thin asphalt paving!).

I have 20+ years experience as a coatings formulator.

I agree that that car cement was probably a petroleum based product -
probably the residue remaining after refining that's called "tar"
in my part of the country (like in Barretts Tarvia) or asphalt. This
material is used to pave roads, make shingles and water proof roof
seams.

At least some tar is solid that has to be melted to be applied but
it can also be dissolved in a solvent like turpentine, allowing the
it to be applied at any viscosity and suitable for spraying.

I'm sure the railroads used the minimum amount of car cement to save
money. The layer of car cement may be a little thicker than paint
but it's not likely to obscure any details. Some railroads only
applied car cement to roof seam caps only.

AS for the dulling of the car cement over time it may be oxidizing.

Car cement looked pretty permanent though and I've seen some picture
of cars repainted over the car cement.

Ed