I would outline the layer doing a CTRL+Click on the thumbnail, then Select | Modify | Expand, about 5 pixels, and paint with white on a new layer underneath the alpha. I think a stroke just outlines, but the Select | Modify | Expand will fill in the entire shape of the letter + the expanded pixels, so it will fill in areas underneath. I don't know this alpha, but I get slightly different looks using a stroke and the expanded layer depending on the texture and look of what I'm wanting to fill in.
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Sarah 
Equipment: Canon T2i (550D) with 18-55mm f/3.5-5.6, 55-250mm f/4-5.6, 50mm f/1.8, and 400mm f/5.6L lenses
Software: Windows 7, PSE 6.0 (Editor and Organizer), and PSCS 4
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