Q. The gray paint on my deck is peeling and has a yellow color underneath. Why is this? Can I just paint over it or will the yellow keep coming through?
A. The yellow color may be the chemical used to preserve pressure-treated wood.
I would not paint directly over the peeling paint. I would recommend that you remove as much of the loose paint as possible with a wire brush, then use an orbital sander to remove the rest.
Once the paint is gone, you could give the deck a final sanding using a 150-220 grit sandpaper.
Clean the deck of excess sawdust and debris — using a power washer would be best.
Now the deck is ready for you to apply a wood primer and finally, paint. This may seem like a lot of steps to remedy the problem, but if you paint over the existing peeling paint, the new paint will start peeling at a much faster rate and you will continue to see the yellow color.
A painted surface works best if it is properly cleaned and primed first. Good luck.