The PO of my baja, let it sit in a river most of last season and didn't clean it before he put it up for winter (northeast). I can get most of the grime off, but the part of the hull that was under water is coated with cement!!! I'm about to take a sander too it. Suggestions?

the works bowl cleaner from menards

There is a product called on & off. Ck at West Marine if u have one near you.
Norlo is an acid that works.

What about muratic acid?

Muratic acid won't kill my gel coat? If it won't I've got some in the garage. Gloves and paint brush?

I use muratic to take the rust stains off my hull could you take a steam cleaner to it / pressure wash it, then hit it with muratic?

Wise idea with On and Off, muriatic acid and the other cleaners is long chemical gloves, long shirt and goggles. If your skin is sensitive; more facial protection. Don't breath the fumes too long either! Good ventilation is important.

When done flush the area with copious amounts of water.

I have seen people use plastic scrapers, but have read articles not to use an edge, but only a very stiff brush. Not sure where a green scratch pad would fit in the mix.

I would question if all the stain can be removed, but at least you should be able to get to a smooth finish.

Should you decide to paint (and not use bottom paint) keep in mind the PPG, Awlgrip and Imron are NOT suggest for any part of the hull which will be in the water for more than 16 hours. Curing seems to extend the time, but still, it is something to think about.

From what I am understanding PPG is working with a formula to give a longer "water time" but I don't think it is available yet.

Pettit has been promoting the Vivid line as being hard, smooth and a great antifouling product, but the people I know who have used it have found it has endurance issues.

Boat has been cleaned and scrubber for hours by me. If I can get the hull bottom smooth and the stain gone from the side, I'll be happy. So I'm going to paint some acid on and let it sit a while

I mean to use the scratchy/very stiff brush with On and Off, muriatic acid or the other cleaners.

For these products to work, they must get "into" the surface of the "crud". If there is now little or no porosity to the surface, it might be even harder to get it off.

Only partially off point... There are studies being pursued about how much faster a hull that is not perfectly smooth is over one that is.

I will post it if I can find it again, but I believe the experiment was being performed in NZ with what is basically a near non-skid type finish on the bottom of a boat. The coarseness creates air and less friction than water. Sort of the same discussion from a number of years ago about symmetric or non-symmetric dimples on a golf ball... or dimples at all!

Are you trying to convince me too keep it dirty???