Don't know how many of you have done this, or are thinking about it. Thought I'd share my experience here.
Overall I'm VERY happy with the kit and result. The instructions could have been more explicit, but with a little searching on the web you'll find a few more detailed versions and stories.
Step 1: Marine Clean - super degreaser. Removes gum/varnish etc. Basically you mix this with boiling hot water 1:1 ratio (makes 2 litres total) and dump into your tank. You can use your petcock on this step. I didn't have one, so I sealed my petcock with a chemical-proof rubber glove I bought years ago. Cut the pinky finger off and then cinched it tight around the petcock mount by twisting wire around it. Don't know what I would do on a different tank with the hole for a petcock to bolt to.
You swish it around and let it sit in different positions for 24+ hours. I did mine for 30 hours. Don't neglect the top of the tank. Let it sit upside down (seal the filler hole) for a good long time too.
Drain and flush out with water. Get it pretty dry after this.
Step 2: Metal Prep - acid. Rust remover / zinc coater. This step is quicker - 30-45 minutes. Seal up the petcock again. Dump the stuff in full-strength. Swish it around and let sit like in step 1 but for less time. Some people put nuts and bolts in to knock rust loose. I didn't. If you have a long magnetic tool used to fish parts out of engine blocks, then you'll be in good shape to remove stubborn nuts and bolts.
Dump the stuff. It says you can use it again. For the $10 the bottle costs, I wouldn't bother. What came out of my tank resembled coffee.
Rinse rinse rinse. Then DRY DRY DRY. Lots of moving air. I used my shop vac on blow for a while, and then my wife's hair dryer on and off for a couple of hours. At times the tank was hot. You don't want ANY moisture in there at all. Don't trust your eyes. If you think it's dry enough, put the hair dryer on for another 30 minutes.
Step 3: Liner - Tiny little tin of what looks like silver metallic paint. Again, seal the petcock. Dump in the liner - make a paper funnel for this out of construction paper or something. Snip a hole in a coffee filter if you have one. Whatever the paint touches will have it on it forever. Seal the filler hole. Rotate the tank slowly to cover all areas. Keep doing this over a 40 minute or so period at least. You'll have to drain the excess. This isn't easy on some tanks (including my 550) because the floor of the tank is flat, and the petcock has a raised area around it.
One thing. I wrapped my entire tank in plastic before doing Step 3. The tank was well sealed off except the filler hole and the petcock. Masking tape is good for the liner sealing. No rubber gloves needed to seal here. Don't put masking tape on your tank directly - you're likely to peel your paint, especially candy top coats that are 30 years old.
This was important as you will invariably end up with the liner on your fingers, and you have to handle the tank a lot. If you get the liner on anything, including the exterior of your tank, it will be there FOREVER. I painted the lip of my filler hole with the stuff as you can see in the pics.
Here's a couple of pics of the finished liner. Sorry I didn't get more 'during' pics, but Steps 2 and 3 are constant activity type things.
Allow 3-5 days for the liner to cure before putting fuel in it.
Do this in a WELL ventilated area (Step 3). The stuff emits some serious fumes for days.