Is it ok to use ready mix joint compound on new drywall as a thin coat or skim coat ( if that means the same thing ). I just want to paint the walls or what is best to seal drywall for just painting. I’m just a DIY. Thanks
Joint compound will not seal drywall, which is what the likes of PVA primer does. It's essentially as porous and absorbent as the drywall itself, meaning you'll need two coats of paint instead of one. If you need to level imperfections or get a Level 5 finish, sure. It's not a substitute for primer.
Purpose-formulated primer is quite cheap and will be much easier to work with than drywall mud.
Joint compound is not primer!
I'm unclear what you're asking (and maybe you're unclear what you're after).
For a joint compound to be used to level the surface, essentially like bondo, sure, that's fine. It's a perfectly viable sanding filler. But to prime the surface in preparation for topcoat? No way. You wouldn't prime your car with bondo.
But yeah, once the surface is level, hit it with a primer then topcoat. You'll do fine.
But it's just DIY
Either do a rubbish job, or do a good job. Your call DIY isn't necessarily about shortcutting/doing shabby work. It's often just about saving money on labor, in which case you want to do the job tip-top to the limit of your abilities.
In general if your drywalled walls have been finished - means the gaps are filled and sanded then no you should not be adding extra joint compound as a DIYer.
Adding more compound - especially the ready mix which is too thick and dry - is basically ruining the flat look of your walls. You should only be using it to fill divots or major flaws.
There are professionals that skim coat entire walls to have a truly "flat" wall. These are very high end services that most professional drywallers can't provide.
What will happen if you try to skim coat your wall?
To make things not have an edge, you will probably sand all of the skim coat off.
If you lightly sand it you will notice ridges and edges even after painting.
What should you do?
Put on two very thick layers of PVC primer. If you are finishing with an oil based paint your second coat should be a primer made for oil based paints.
So to answer your question...
Yes skim coating is OK if you want to spend an ornate amount of time doing it, you have practices the technique many times and you have the innate artistic ability to do the job... along with the tools. Here is an example of a Level 5 grade install and how this guy goes about it (uses a roller and really thin joint compound - not ready mix).