I cleaned the almost 20 year old gas out of the tank. Looked like mud. Cleaned it with hot water, thinner, simple green and screws the loosen the rust up.

Then it has sat for about a month empty and I see some rust inside the on the wall now..not the bottom but I obviously cant see everywhere.

Should I but one of the Tank Sealer kits?

Or just clean it again and put gas in it.

thanks
Rc

buy as large as possible (large as will fit) inline filter - get a clear one so can easily see if it is gunked up down the road.

after my recent experiences based on what you are saying my vote is NO to start with.

I had sealed two tanks 15 years ago with great success.  I have also heard several horror stories of it coming off in sheets which I never understood until my last tank.  Then I cleaned the tank on my 32 year old enduro.

Here is my suggestion.  You are describing a slightly rusted tank. We used the kits in the shop only when we had an almost perforated tank or a leaky tank.  If you follow the kit you will use an acid etch to clean the tank.  That is the key step. this disolves the rust.  from there you neutralize it and Kreme it.  Now if there is any issue with that Kreme adhearing (such as a flash rust or residual rust because you didn't get all of it out) it will come off in sheets.  I can attest to that.  I did a large tank with baffles and had it looking beautiful after the acid wash.  Then I found there were surfaces on the top wall of the tank and baffles I hadn't gotten totally clean.  The Kreme then loosened from those and came down in big sheets.

Next when I got to my enduro tank, I asked the advise of the other mechanics I used to work with.  Here is what they said.  Get a bottle of Muratic acid and mix it 1 part acid to 4 to 6 parts water.  This is the same type of acid you get in the Kreme kit. (only stronger like it used to be 10-15 years ago) acid wash the tank but only let it soak for 20 minutes or 10 minutes either side of that to when it looks clean.  Use a flashlight to see.  The key is to remember you can only remove the rust where the acid touches.  Once you pour that out then rinse it with hot water very thuroughly and then use a about 1/4 cup to 1/2 cup acetone swirled around the tank to remove the water (you can also use isoprol alcohol but it is not as strong a water binder)  Pour that out.  At that point that tank will look dynamite.  Imedeately fill it with gas.

Now some very important points.
!. ACID will attack other metals...remove the petcock and make a plate to go over the opening..Use a piece of inner tube rubber as a gasket (bike or motorcycle).  .I don't rely on duct tape.
2. Protect your eyes, face and skin.  Goggles, faceshield and rubber gloves a must.
3. COAT THE ENTIRE TANK IN VERY THICK COAT OF TURTLE WAX FIRST...DON'T RUB IT OUT..JUST COAT IT.  This will help protect it should you spill some acid or worse yet on the paint is acetone.  
4. KEEP THE ACETONE OFF THE PAINT!!!  IT WILL SOFTEN IT.  You can use isopropal alcohol as that is much safer.
5. Lastly, if you wait around after the hot water rinse before the acetone or isopropal you will get flash rust.  not a big deal as it is more cosmetic than a problem.

Last tips....

use the block plate on the pet thingy as described.  leave the top open during the acid .  do this out side for fumes.

when you do the hot water rinse pull the plate and let the water flow out.  Now dry the outside and tape up the petcock opening with duct tape...make sure it is stuck well but leave a pull tab.  Then throw in the acetone or isoporpal and tape up the filler opening.  Roll it all around and then rip the tape off the bottom and pour it out through the petcock opening.  Watch the acetone at this point any that runs on the paint on the bottom of the tank wipe it up right away.

Lastly then pull the duct tape from the filler, slap the petcock in it and fill it with gas.

Done.