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Just to add, if you don't have problems with the surface and it's not a bare redwood then sanding and Zinnser (bulls eye) 123 http://www.zinsseruk.com/product/bulls-eye-1-2-3/ Would be my choice. Just stir it before and during use because the solids settle quite quickly. It's water based and cover stain is oil based so it's easier to clean up. Otherwise cover stain and wash the brushes with white spirits.
B.I.N. is effective but it's a bit harder to work with.
Great thanks folks. I'll pick up the Zinsser cover stain and report back. @Airhead you mean the Zinsser B-I-N product right?