Black spots on newly finished gate slats

[Weezy1905]

Hello, brand new to the forum. I have been interested in getting in to woodworking and am gathering a number of tools to put a small shop together. My first project was to revive the old planks in my side gate. I'm almost done and thought they were looking pretty good, but noticed black spotting happening on some of the newly refinished pieces. I was hoping someone might know why this is happening? The first picture shows the last 4 or 5 I did compared to the old weathered planks and they turned out great. The second picture shows a plank from another section that is showing these spots.

I sanded each plank down using 80 grit, didn't feel it was necessary to go beyond that since these are outside. Then applied two coats of Thompsons water and stain sealant. Should I have sanded up to 220 grit? Is it the sealant? Any thoughts would be appreciated, thanks!

   

   

   

[iclark]

Going to a finer grit sandpaper will still not get the mold and mildew that is down in the crevices, cracks, and divots.

My guess is that the boards with the black spots spend more time in the shade than the others.

My guess is that you need to use a good deck wash compound and let it dry well before applying the finish.

Also. welcome to WoodNet.

[Weezy1905]

Going to a finer grit sandpaper will still not get the mold and mildew that is down in the crevices, cracks, and divots.

My guess is that the boards with the black spots spend more time in the shade than the others.

My guess is that you need to use a good deck wash compound and let it dry well before applying the finish.

Also. welcome to WoodNet.

Thanks for the input, appreciate it! I'll give a wood cleaner a try and see if that does the trick