Hi all, I’m into my build of an M813 5 ton truck and I sprayed the fenders wheel arch/motor assembly after the frame.
I used Vallejo Air and although the frame dried great, the fenders dried a weird crackly type of finish. All parts were washed but not primered.
What am I doing wrong?
Not sure about your resin or paint, but I always use a decent rattle-can primer for auto parts (Halford’s grey) and it sticks like magic.
My guess is the washing wasn’t enough. I know some put all resin parts into an ultrasonic cleaner. I think wiping them down with rubbing alcohol is good. Resins vary in composition. Some are not as thoroughly cured and some get delivered with a stubborn coating of unpolimerized resin. Also… mold release agents can vary. Some require a more thorough cleaning. Best to do all this before assembly.
Resin should always be properly primed before painting. This is especially important when the paint is acrylic as you are basically spraying ‘water on oil’ when you don’t use a primer.
You should also wash the resin in warm soapy water before assembly, and again after handling it. Then when dry prime it.
Thanks PetBat, what primer should I use under Vallejo Air?
Thanks for that Peter. I’ll clean and wipe down as suggested before I move on.
Any primer for modelling should work, or at a pinch a primer for automotive industries, although personally I think the latter are too thick for some of the more intricate parts on a model. I prefer Tamiya Superfine in the rattlecan as my go to primer. I tried Vallejo’s and Mig’s but did not like it myself.
Many people prefer a type you can use through the airbrush for better control, but I find Tamiya rattlecan works fine for me as long as you take care to only lay light coats and allow each to dry before adding the next. It is a very thin consistency and I have never suffered loss of detail doing it this way. Also being ready to use, there is no problem with getting the right mix of primer and thinner and no clean up to contend with.
Thanks Peter
I havent tried Tamiya primer in a rattlecan, but I’ve got Stynlrez and several Vallejo primers. I’ll try and get that first coat of V Air off the part, then give it some coats of Vallejo primer (the Styn…… is black which isnt my favourite).
I used Tamiya matt coat in a rattlecan earlier on the M 813 build and it happily covered up all the weathering I’d done on the tires. Silly me.
Great advice on cleaning the resin parts. I’ll get the alcohol out tomorrow.
I build a lot of resin kits - 15 1:35 armor and artillery kits in the last year. Resin should always be scrubbed with a toothbrush and dish soap in warm water before assembly and primed prior to painting. I’ve not had any luck with any of the acrylic primers on resin and threw them all out. I have had good luck with Tamiya (lacquer) primers, automotive sandable primer (lacquer), Krylon Colormax flat back primer+color (enamel), and my favorite Mr Color 1500 Black (lacquer). All are rattle can. All yield good results. I prefer the Mr Color as it dries to a thin hard coat and is available in black. The Tamiya is just as good; but, not available in black. I have some Mr Surfacer 1500 black in the bottle that I use in the airbrush to get into nooks and crannies not reached by the rattle can; but it is rarely needed.
I miss Floquil primer
When finished, don’t forget to turn the can upside-down and spray a quick burst in order to clean-out the spray tip. 
The beauty of Tamiya Primer is you never have to do that John. I did that originally, but forgot one time and everything was fine when I went back to see if I’d gummed it up. I stopped doing it, and in all the years of using it since, never once has the nozzle been blocked.
It’s because you Aussies are already upside-down. 
Lol!
No, we are right way up! You just have the wrong perspective from where you are… 
I envy your skills and thanks for the advice. Im on my first resin kit, using Real Model’s M800 tractor conversion to make an M813 5 ton truck. The radiator grille was badly molded with the top rail of the grille swallowed by the ermm, mold block.
It broke up as I tried to separate it and now ive a several-piece grill to rebuild.
Like all things, resin kits become easier with experience and practice. The quality of the resin kits is far more variable than styrene. The best resin kits are very well molded, detailed, and easy to remove from the pour plugs. There are other kits that are almost impossible to build - voids, pin holes, short shots, warpage, uneven shrinkage leading to major fit issues. I’ve got a couple of kits that I keep pulling from the stash, examining the parts, and then putting them back on the shelf while hoping that another manufacturer will release a kit of the subject. But, there are an awful lot of subjects that are only available in resin. So, build a few. Each one will get a bit easier.
Rick
Yes, I get your point about varying quality. Ive got Real Model, Hobbyfan, Black Dog and PSM resin kits on the shelf. The latter, a tank and pump unit looks very nice…….
I managed to piece the truck radiator grille back together, glad I had more types of c a glue!
I’ll truck on with this build, it might be one of those 3 foot specials (looks OK from 3 feet away). Then I’ll go back to styrene for a break, but I will tackle another resin kit after that.
So . . . polyurethane is “oil” but polystyrene isn’t?
The second paragraph is far more important than the first. If the “primer” does not have an active solvent base to chemically bond with the polyurethane, it’s just another layer of paint.
KL
Interesting topic. I used Vallejo sand primer on the resin hood of the M813 then sanded it gently. Then later blew some Vallejo Air IAF Sand over top.
It’s dried out to a smooth, glossy finish.
Dont know what’s waiting for me in the booth tomorrow but I’ll post relevant feedback.
ekim—
!etam sreehC