You wouldn't need cement board if the surface is somewhat even and if the thinset sticks. I would spot grind the surface in a few places to help adhesion if there is no exposed concrete. You can get tile for a buck or less per sq ft. With thinset and grout you may be at about $1.50 per sq ft if you do it yourself. There is a good chance you would be fine doing nothing but you would not have peace of mind. My wife would not eat anything grown in tar but she worries about everything. If thinset sticks you could probably skip the tile and just stucco. I use Rapid Set stucco mix available from Home Depot sometimes as a sealing coat. It is not a portland cement based product. It has almost no shrinkage, a short work time and is very waterproof (resistant). I doubt it would react with the tar coat as most sealing paints would. Zeuspaul

Thanks for the exposee on paint components, guys! I'll try the "Latex/Acrylic", Bailey 09. We finished the pour today, I'll get a picture when the form comes off. Laura, I'm leaning toward a huge flower or two, orange Montbretia? Streaks of red-orange and black freckles, once I've tested the "latex" wash. We can sit like drunken bees and gaze upon the glories! Maybe I should use white portland/sand for the top coat? I'd be coming up way above that rebar, Dena, if I can pull it off. My helpers were excited enough to cheerfully put up with a zillion mosquitoes in the inside-of-a-dog's-mouth air, but he's going for solar energy engineering. She is very young, but we're happily sharing Jim Hensen nuts and bolts and she lent me a book "The secret art of Dr Seuss" (great!!) which I'd never seen, so who knows.. (I now feel totally justified getting a weeping Norway spruce) Here's an April view of the location, not the view, but you get the picture. The couch is now over on the left, where you can just see a wood bench on a levelled spot. Those trenches still have to be "laid" with dry mix, heigh ho.. There's a glimpse of the pond on the far left. Here is a link that might be useful:

Bead blaster concrete surface prep. They can be rented.

Hello w0lley32, I have worked in the ink and coatings industry for 36 yrs or so and have experienced tiny bubbles forming on patching compound after sealing them with latex primer or even after a top coat with latex or acrylic paints. This may be caused by "off gasing". Because the patching compound on a smooth surface is applied in very thin film it is subject to quick penetration. Components in the patching compound react with resins and or solvents in the latex paints to form gases which bubble up through the surface of the wet paint. Even water base systems have evaporating components. For sure using an oil base Alkyd primer will prevent the bubbling but if you top coat this with a latex or acrylic (water base) paint or primer it may penetrate the alkyd and off gas again. The sure way to prevent this is to top coat with Alkyd paint but it is clear you do not want to use these products so another avenue may be to patch the bubbles with a different type of compond such as Polyfilla that dries much harder than conventional drywall compounds or even white wood filler. I've used both and had good success. You can patch a small area first ,sand and clean and test it for bubbling after you prime with a good latex. For sure you need to find a patching compound that dries with a harder finish to prevent latex penetration and off gasing, otherwise you'll get stuck using Alkyd.