The Zep purple degreaser is good stuff. We use it to clean grease bins at some of the grocery stores we pressure wash. I've seen that stuff eat the paint off of the grease bins. I usually use it full strength, but thats because I'm dealing with built up grease on a grease bin. And yes it does eat up your skin. Although using it full strength does make it more difficult to rinse off, that stuff is suddsy!
Someone asked about the steps to paint. First decide if you want to use automotive or store bought paint. If you are using automotive paint I can't help you much. But if you're using store bought paint then follow a few simple steps and you will be fine.
1) Prep. If you feel that you must get down to bare metal then unfortunately you'll have to media blast. If you only want to repaint over the existing paint (see any vehicle from the 40's and 50's and how many different layrrs of paint you can see) the you need only scuff the existing paint enough for the new paint to bond to. A bonding primer (DTM bonding primer from Sherwin Williams or other commercial paint store) will improve that process greatly. Prior to priming but after scuffing up the paint you need to wipe down the entire are to be painted with M.E.K. (methel ethel ketone), it evaporates quickly and doesn't leave a residue. Wipe with a white cloth until there is no color left comming off of the surface onto the cloth.
2) Prime. Again on the surface there will be scratches from scuffing and sanding, those scratches serve as the foundation of the paint bonding to the previous layer or in the event that you went to bare metal, it will be bonded to the metal. Make no mistake, the condition of the substrate (previous layers of paint or bare metal), affects paint quality. Bigger deeper scratches do not mean better primer or paint adhesion, only a finished product where more scratches are visible. The smother the sufface (yes, even by sanding or scuffing) the better the final product.
3) Paint. I'm not suggesting house paint, or latex in particular. While latex will flex and breathe to an extent, an enamel will be much more durable. Especially since it's a bus and will see plenty of road time: see rocks, sand and other debris from the road. We use industrial enamel on metal and it holds up really well. Unfortunately it's harder to find enamels in flat sheens. Most will have either a satin or gloss finish. Flat is out there, it's just harder to find. You can use a home paint sprayer, I don't recommend the handheld units with the refillable screw on jar. They hold very little paint which is tedius to continuosly refill. It's better to use one of the sprayers that you can get from the bigger retail stores. They generally come with about 25ft of hose and the newer ones can accomodate one gallon cans or 5 gallon buckets. I would recommend a Graco unit. Thats the brand of sprayer. The reason is that there are different size tips that you can use and the paint pressure is adjustable. As for tip size I would recommend a 315. Tip sizes are read as follows: 315. 3 is half of the fan width. 15 is the quantity of paint. So a 315 tip has a fan width of 6 inches, adjust that to your comfort level by buying a different tip. And the 15 is 15 thousandths. If it was 317, then you still have a 6 inch fan but now you have more paint comming out at 17 thousandths.
A comparison: 315 vs 515
315- 6 inch fan with 15 thousandths of paint comming out.
515- A 10 inch fan that still has 15 thousandths of paint comming out but the layers will be thinner because its the same paint spread over a larger area.
So, 315: fan size is 6 inches orifice size is 15.
We typically paint horizontal when the area allows. Overlap your strokes by at least 1/3. If you don't get full coverage, slow down your stroke. If you still don't get full coverage, don't worry, you'll get it on the next coat. You should do at least 2 coats anyways. For a bus I would recommend 3. If you want to sand in between coats (it makes the final product smoother) wait until its fully dry. Oil based paints will clog up sanding paper pretty bad. Don't use too coarse of paper to sand. 320 should be fine. Again you only want to knock down the surface and any runs. DO NOT PAINT IN DIRECT SUNLIGHT! Sunlight will heat up the metal, causing the paint to flash dry. You'll see every stroke from your paint gun and every overlap of your strokes. Paint in the shade. If it's really cold outside, try to wait for warmer weather. Don't paint to late in the afternoon. You want to be done with a couple hours to spare before nightfall. The reason being that when the sun sets, all that metal that was warm during the day will cool when the sun sets and may possibly condensate. I painted some small trash bins once that were outside units. I painted them at about 10pm and got done at around 2am (it's easier to do that kind of stuff overnight when you don't have people and children around to mess up the paint) went back the next day to get my sign off because they were complete, and about 3 of them had been ruined by condensate after I left that morning. This is more of a problem during spring or fall when the weather is still fluctuating so much.
Well, I hope this helps. Good luck.