What's the best stain blocker for pine? We have chalk paint on pine boards and tannins are bleeding through...

H​i! Does your sealer cause whites to turn yellow or cream? We are considering using over chalk paint.

Additionally — Does it pull the tannins in the wood which would show up as a pinkish color?

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Hi xxxx,

Tannin bleed depends on the wood and what was applied to it. Strong tree resins are the best thing to block stains, especially in wood. These are the best type of stain blockers all around when you want something that is permanent. 

Chalk is calcium carbonate which is in most paints, it's what Lime Clean turns into if allowed to dry out. I'm not sure what chalk paint would be technically. If the resin to pigment ratio is not above a certain amount then paint becomes chalky. This is done sometimes to make low cost primers for interior drywall but doesn't work well on wood or areas where a good strong bond is required. 

NanoTech is a crystal clear sealer that can be used over calcium carbonate and the NanoTech doesn't yellow but that doesn't mean the surface underneath won't change color if it was already beginning to bleed. 

The final substrate color is dependent on the wood type and the initial coating. 
To permanently block tannin / extractive / water stain bleed through and keep a nice bright white we would: 

1. Prime with Mountain Pint of White (1-4 coats may be applied depending on severity of knots, staining, wood type and such. Any good Oil based stain blocker can be used but water based stain blockers tend to permit the bleed through eventually). 

2. NanoTech Pint of White x 2 coats or 1 coat of NanoTech and 1 coat of Lime Seal Bright White (depending on what look is desired)

The Mountain would have the best chance at soaking through a "chalk paint" and helping it to bond better. It takes a good strong film to really block stains. We use up to 4 coats on knots or water stains sometimes. Generally, on wood pine trim it only takes one coat of Mountain Pint of White and then NanoTech Pint of White or Lime Seal Bright White makes a nice white paint. 

When coming back to these stained areas, years later, this method, relying on tree resins to block the stain, is the longest lasting and most reliable. When it comes to stain blocking, some stains are more persistent than others. Knots, exotic woods, cedar, redwood, water stains... these are the most troublesome and so we would apply the most coats to these areas. Regular pine trim or oak usually doesn't require as much attention. 

If a wood stain or an off white color or a mid to deep tone is being painted on then all of this becomes much less important. These stains require the most attention when they are going to be coated with a white finish coat.

Best wishes,
Tom