Several issues:

1) There is a chance that you misunderstood what he was suggesting.
Not likely, but possible.

2) Cans of various paints and/or primers cannot be arbitrarily mixed
together without knowing the chemistry of the products. For example,
two leftover cans of white interior latex paint can't always be mixed
together. This is 99.99% safe if the paint product is from the same
manufacturer and the same product line, etc. But randomly mixing
paint products can produce a gallon or two of worthless crap.

Remember that there is a lot more to paint chemistry other than
"latex or non-latex." For example, so-called water based paints generally
have about 50% (?) organic solvents blended with the water. And all
paints have non-generic solvents blends, emulsifiers and binders. If
paints and/or primers are blended, there is often the very high risk that
the blend will perform poorly or not at all.

If you blend incompatible products and you are very lucky, then shortly
after the mix is stirred the pigments and binders will precipitate to the
bottom of the can in a rock hard mass with the solvents floating on top.
If you are less lucky, you won't discover that you've produced crappy
paint until some time after you've applied it.

3) Even if the chemistry is compatible for the paint and primer that this
painter is suggesting, there must be a reason why the manufacturer
didn't do this at the factory to save painters or homeowners time, effort
and money.

4) Many/most "professional painter" aren't. I haven't hired anybody for
painting in over 20 years just because there are so many hacks out
there. When my son was about 13, I realized that he was actually
a better painter than anybody that I had ever hired. (Thank you Boy
Scouts of America for teaching useful life skills!).

The good painters are expensive, but expensive painters aren't always
good. I learned that the hard way. And references don't mean crap.
The average homeowner doesn't know squat about a quality paint job
and isn't a reliable source of references. I've discovered that I am the
only homeowner on the block ambitious enough to climb on a ladder
to inspect second story prep, priming and painting. The only one to
walk a roof and inspect a roofing job before handing over a check. Etc.

Good luck,
Gideon

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TC wrote in message ...
My mom is having her home painted. The guys who gave her the estimate
says he's going to mix the primer/sealer in with the paint and then
paint the house. This does not seem like a good idea. There's a reason
the primer/sealer goes on first. Or is this ok and done professionally?
If it's a bad idea what things can happen?

It's going over CBS (concrete blocks).