HELP! My (ex Tom) cat keeps "spraying" all over the house. What can I do to stop this?

The cat was castrated when he was around one year old. As he gets older (he's now 13), he gets more "aggressive" with his spraying. I used to think it was because he was scared, ie saw another cat outside, was marking territory...BUT he specifically sprays MY stuff (photographs, books) seemingly when... show more The cat was castrated when he was around one year old. As he gets older (he's now 13), he gets more "aggressive" with his spraying. I used to think it was because he was scared, ie saw another cat outside, was marking territory...BUT he specifically sprays MY stuff (photographs, books) seemingly when I'm not fast enough getting his food or letting him out (he has a clean cat box, plenty of fresh dry food & clean water) OR (especially) if I leave him inside for long periods of time (more than 4 hours). His sister does NONE of the above.
A cat door is out of the question (there are more than a dozen cats in the neighborhood; the house would be Grand Central Station!).
Is there anything I can spray to discourage his spraying?
Right now I'm putting them both outside when I have to be gone for most of the day.
This is the only (of many) cat(s) I have had to do this.
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