How to Paint Tongue Grooves


Tongue and groove is a way of cutting the edges of milled lumber so that each piece interlocks with the next board. You can find tongue and groove lumber in flooring, siding and even ceilings. This type of lumber is used to increase the strength of the sheathing, while also acting as a vapor barrier to exterior moisture. If, for some reason, you feel that extra moisture will build up on the outside of the tongue and groove planking, some extra precautions may be in order.

Things You'll Need

  • Saw horses
  • Exterior-grade oil-based primer
  • 2-inch polyester trim paint brush
  • Empty one-gallon paint container
  • Wooden stirring paddle
  • Medium-grade sandpaper
  • Hand-held sanding block
  • Caulk gun
  • White oil-based caulking tube
  • Place all the wood on a pair of saw horses. Examine the tongue and groove of each board and sand any rough spots that you find.

  • Clean all grooves with a paintbrush to remove all dust and sawdust. Brush out the tongues, but these areas should not require as much cleaning as the grooves.

  • Paint a tongue and groove from different boards with liquid from a well-stirred can of oil-based primer. Let the two boards dry.

  • Place the tongue into the groove to fit the two boards together. If they don't fit together, thin the paint with a small amount (about 5%) of mineral spirits and paint two more boards. Mix the paint and mineral spirits together in an empty one-gallon paint can.

  • Paint the remainder of the boards with the oil-based primer. Spread the paint quickly and evenly with the brush.

  • Let the paint dry and then fill any cracks in the wood with an oil-based white caulk.

  • Install the tongue and groove and planking after you have prepped and painted all the stock.

Tips & Warnings

  • Always use a high-quality polyester or bristle paintbrush to spread the paint on the milled lumber.
  • Be sure to cover every part of each tongue and groove, and then spread the paint onto the face of the board.
  • Allow quick drying to the touch. Shellac-based primers need plenty of extra time for the paint coat to harden.
  • In some situations, such as outdoor use, it may be advisable to prime the whole board.
  • You can use shellac-based primer instead of oil. Just remember to thin this type of paint with denatured alcohol, if necessary.
  • Paint quality may vary among the different manufacturers of primer paint.
  • Do not let the paint puddle and build up on or in the tongue and groove slots of any board.
  • Always use an oil or shellac-based primer in a well-ventilated room. You may want to use a paint mask and filter when working with these types of paint.

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  • Photo Credit porch image by MLProject from
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