What to do when ?

It sounds like you need more mud. Do it in several thin coats, slowing building it up in the middle to cover the tape and feathering the outer edges.

Hello Jeannie! Yes you can mud over it again and sand once it's dry.

Mud back over it and sand it again.

Yes. I would suggest rather than traditional sanding use a wet sponge. They make them specifically for this but I have always used a car wash sponge. It cuts the dust particles in the air to nearly nothing. Normally, it takes 3 passes of mud and smoothing to get the seams perfect in my experience.

Hello. Yes mud over it again.

Sometimes it takes several applications with it drying overnight to get the proper finish. For optimal results somethings cannot be rushed along.

Mud over. Then lightly sand the edges. Paint won't cover and you will be sad everytime you look at it knowing you didn't take the time to do it right.

Yup. Just mud over it. Smooth it out as much as possible. I like to use a damp sponge rather than sand to do a final smoothing. I just put on enough compound to cover the tape and seams. I also use mesh tape rather than paper tape.

Hi Jeannie, sorry you sanded too much, yes mud again, let it dry and sand a bit less. Good luck and stay well.

Paper tape tends to tear. Needs to be dampened before being applied and can bubble when too wet. Needs an initial bed of compound to be embedded in. Then more compound over it. More work to smooth out. Seams can easily tear over time from house movement. Cracks can develop.

Mesh tape doesn't need an initial bedding compound to be embedded. Easier to work with. Sticky on one side so just apply to the seam. Compound directly over the mesh with less compound for a smoother coat. Does not tear and limits seam cracks with future movement.

I use a car sponge. Brand doesn't matter. It's tight with no holes for a smooth finish. You will need to rinse quite often as it will get clogged with compound. Use light pressure and you will see the compound blend smooth.

If you have bumps or ridges in the dry compound use a drywall blade/knife and one sweep over the seams to scrape them off.