I have patched and sealed a couple patches in the floor.

Now the bottom of the car is the bottom of a 50 year old car. It's dirty/oily.

I rough sanded things down a bit around all the sealed patches and I'm getting ready to seal it up.

No matter how many times I wipe, there is some residue from that original factory undercoating.

My questions:
Should i spray the zero rust onto the undersurface first, then apply duplicolor rubberized undercoat after wards to blend the texture.
OR should I spray the rubberized undercoat first since it (may/may not) stick to a less than perfect surface and then even it all out with zero rust?
Adhesion Promoters such as SEM77723, will this help, hurt or do nothing?

As it is, I'm scrubbing with industrial purple. wiping with parts cleaner and then using pre-paint prep/ W&G remover prior to covering.
There are blobs of original undercoating that are still there.
I have zero intention of smoothing out the bottom of a street ride. Under the drivers feet isn't important to me.