These are generally non-toxic and biodegradable, and can work outside as repellents for cat urine on the porch, as well as keeping cats away from your property entirely.
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Citronella oil is the home made cat repellent I had most success with. Although best known as a mosquito repellent cats find the Citronella odour very distasteful. Mix 1 part Citronella oil to 4 parts water and spray liberally in the trouble spot.