How many gallons of coating – paint, stain, etc. should you purchase for your project? The answer is simple – you’ll never know until you try.
When purchasing for a project, the coating material (hereto forward simply “paint”) is typically the hardest commodity to estimate. There are many variables to consider. Be prepared to either go get more mid-project or shelve ample leftovers for future touch up.
The estimated coverage shown on a can of paint, e.g. 400 SF, comes from a test performed somewhere in or near heaven. If you can get paint to go that far, congratulations. I think those tests are run on a perfectly clean smooth non-porous surface with optimum temperature and humidity. They must also be going over a color with the same exact color.
For those of us who have to deal with the real world, take this approach. Measure the area to be painted. The gross area can be quickly estimated. For walls of a rectangular room, add the length and width, multiply by 2 and then multiply by the height, typically 8’. This gives you the overall square footage. Now consider the number of doors and windows. Are they significant enough to reduce your number? Just ignore them to account for the generous claims on the can. Divide the gross area by the advertised coverage. Buy that many gallons of paint.
But it can be more art than math and science. What is the texture? Rough surfaces will require more paint. What is the color change? Going over graphics or making a strong color change will require more paint, perhaps multiple coats. Is it a hot exterior job? Is the surface porous? Bare wood or other raw surfaces should be primed. A good start is to use a stain-killer/primer such as Kilz or Zinsser. It takes more time, but is well worth the investment. If you try to paint a water-stained ceiling with white ceiling paint, be prepared to do 4 coats before you give up and put on a layer of stain-killer.
If you are doing two coats, do not simply double your estimate. The second coat will not take as much paint. Yes, it should, but you will spread it thinner, miss spots and not trim as close because… because it’s one of those things humans do.
How thick is the paint? An oil-based stain for a deck may be thinner than water and spread far. Conversely, the wood may soak it up requiring heavier, multiple coats.
There are some very thick deck paints on the market now. Home Depot carries the Deck Restore Deck Restoration product which is quite different than conventional paint. They also sell Behr DeckOver, which is a step between the extreme resurfacing of Deck Restoration and traditional solid stain.
A gallon of DeckOver is advertised to cover 50 SF with 2 coats. Here comes the tricky part. How do you figure the full surface area of the deck shown above? Does anyone care to calculate the area of all the sides of the rails, pickets, stringers and steps?
For this job, the contractor started with a 5-gallon bucket. It is a substantial cost savings over purchasing 5 individual gallon cans. Basic calculations and reports of the requirements of the previous recoating would indicate that 5 gallons was well more than enough. After the 5 gallons was gone, two more gallons were required to finish the job. At least they didn’t buy two 5-gallon buckets at the start.