sandable primer over Epoxy?

My answer to your question is yes. However, I don't know why you would want to bother for the cast. By applying surfacer within the epoxy time window you can avoid having to reapply epoxy at a later date if you cannot topcoat within that window. That's fine for the sheet steel where the surfacer, as intended, is going to be sanded anyway prior to topcoating. I don't see what you would gain by doing that on the cast. As with any cured product, the surfacer should be sanded prior to applying anything on top of it - even if done at a far later time. If you don't use the surfacer on the cast and (for whatever reason) go past the time window of the epoxy you should scuff with a Scotch-Brite hand pad and apply another coat of epoxy at such a time that you can apply the topcoat direct to the epoxy within the time window. Either way, you are looking at applying another product before the color and a sanding or scuffing operation. Application of the other product is just at different times; e.g. within the window for the surfacer and after the window for the second coat of epoxy. Sanding or scuffing should be done in either case. So there is no benefit to the surfacer unless, as CNKS says, you just like sanding and don't mind going to extremes in trying to make the cast as smooth as the sheet metal.

I don't know why your jobber is trying to steer you away for epoxy. Perhaps because he sells more etch primer. I haven't bought any etch primer in years but suspect it may be cheaper that epoxy. In any event it has a time window, just like the epoxy does. And usually more restrictions on the use of it. Etch primers and surfacers go together almost as a requirement whereas epoxy can be topcoated directly without any additional priming. There are those that prefer an etch primer and those that prefer epoxy. For me, it's definitely epoxy. I wouldn't give you a plug nickel for any etch primer. If for some reason, I was going to etch-treat bare sheet metal, I'd use the separate, two-step chemical clean plus conversion coat method. That's a rather messy and time consuming process and I can see why it's probably not used to any extent in body shops where time is money and using an etch primer can mean greater profits. Especially since a surfacer is likely to be used regardless.

Don't let recoating epoxy after the window has past be a key factor in any decision. I think people make too much of that. It's not as bad as it sounds. Much of my work consists of sandblasting as an initial step. I prefer to apply epoxy the same day for the best protection. As CNKS indicates, Picklex 20 teatment is another option for interim protection and I have used that too when it fits my schedule better - but I still prefer the direct epoxy right after blast. My effort may spread over several days because I also do several smaller parts at once before any color. I sometimes miss the window since I paint outside and the weather doesn't always cooperate with my plans, despite weather forecasts. I just scuff when I am ready to apply the topcoat, shoot another coat of epoxy and then proceed directly to either the color or surfacer (if needed) depending on whether the part is sheet metal or cast. For me, it's not that big of a deal to do that. Pro body shop materials, procedures and equipment are not always the best choices for a DIY working on a tractor. Big, big difference between a DIY doing a tractor on his own time (and budget) and a pro shop doing collision repair on a car or truck for profit. There are some commonalities to be sure, but they are two quite different worlds.

Rod

Nason jobber steered me to their Fast Dry as a cheaper non-iso product. So, if I stay with Epoxy primer I do not need supplied air. Looks like if I use a typical surfacer it will also have additives that contain isos. I read Rod(NH) photo thesis 'Painting the B Fender' again and noticed the PPG surfacer he used has isos. I guess that makes sense since they need to harden fast to allow you to sand em.

Your NASON jobber was correct with regard to the Fast-Dry. You basically have two choices for a non-iso solution in the DuPont automotive lines: NASON Fast-Dry in the economy line and DuPont Centari 1K mix in the more expensive category. Both are acrylic enamels. In PPG the choices are OMNI MAE in the economy line and Delstar (if you can find it at all) in the more expensive category. These are acrylic enamels also. Both DuPont and PPG have a non-iso surfacer. In DuPont, it is called 131S Fill-N-Sand. In PPG it is called OMNI MP181. Both are acrylic lacquer surfacers as opposed to a urethane surfacer. Neither requires any activator or hardener. They dry very fast and are easy to sand. You won't many body shops still using them because they are old technology just like lacquer topcoats and single stage synthetic and acrylic enamels. The urethane surfacers have advantages for body shops but the lacquer surfacers are still quite satisfactory for those wishing to avoid isos and take the time to understand the issues when using them as opposed to the urethanes. I used the 131S a lot right up until about four or five years ago when I moved to the MP182 urethane.

The epoxy primers I use (PPG OMNI MP170 or PPG DPLF) do not contain isos in the catalyst or in the base product. As far as I know, other epoxies don't either but if you choose a different epoxy product, verify there are no isos involved using the manufacturer's MSDS. If you look to DuPont for epoxy primer, check the time window. The last time I checked one of the DuPont epoxies, the window was 24 hours, much shorter than the PPG epoxies. If you are more concerned with the window length than the cost of the product, look at the DPLF. It is a premium product and has a 7 day window. For some reason that is unclear to me, I have for many years felt that DuPont promotes their etch primers over their epoxies and PPG promotes their epoxies over their etch primers.

Rod

Is the Delstar color with hardener a non-isocyanate product? Or are you refering to the surfacer only? I suppose you could spray the topcoat without the hardener, but would it stand up to fuel as well?

Delstar is PPG's competitor to DuPont's Centari. Both are excellent products that have been around for 30 years or so but are pricey. The use of a hardener is optional in both of them. I was referring to the Delstar only if used without the hardener. I have used quite a bit of Delstar (with hardener) over the years and always loved it. I haven't been able to get any in my area in the last 5 or so years because the local jobbers no longer carry the mixing tints for it due to low demand. Since the auto industry has gone to basecoat/clearcoat urethane, the demand for acrylic enamels has become less and less, especially for the better (and more expensive) ones like Centari and Delstar. There is also a lot of cost competition for both of them in the lower priced economy lines of both DuPont (NASON) and PPG (OMNI) for whatever low-end demand is left in the automotive market. For instance, my local PPG jobber carried Delstar for many years but no longer does. He stocks the mixing tints for OMNI MAE however, a significantly less expensive acrylic enamel from PPG. When I painted one of my trucks several years ago, I wanted to use Delstar. A search of a number of PPG jobbers within a hundred miles of me indicated "no longer stocked". I understand that it is still available in the midwest though. Assuming PPG has not changed the product significantly, Delstar can still be used w/o the hardener since it could years ago. The isocyanates are in the hardeners so once that component is added, isos are then present in the mix. If you want a non-iso product, you have to avoid the use of hardeners altogether. No, it will not stand up to fuels as well as if hardener was used. In fact it may have little tolerance for gasoline spills for several months - and never as good as a hardened enamel or a urethane.

Rod