Post this in the heating and air conditioning forum.

I just experienced the same thing last year. The HVAC fellow I hired said the damage was long term dog watering. The previous occupants had multiple dogs and there was a lot of yard damage from the dogs, so it makes sense. I have a male dog, so built a white picket fence around the unit to keep him from annointing the new unit.

My unit was too small and too old to worry about trying to save, so we(I hired him because he let me work with him to lower my cost---I do have HVAC experience in automotive areas and experience in soldering) simply replace the old unit with a properly sized new one.

He did say the missing fins would cause a reduction in cooling---the reason the fins are there is to promote heat transfer and missing ones reduce that capability.

As far as the warranty, you will have to see if it is covered.

I would think that dog watering would have a peculiar pattern. Was that the case with you, handymac ?

I've seen fins destroyed by dog pee...one of the reasons I
picked my condenser off the ground with concrete blocks
& put the pad on top.
the height of the dog...determines the spray pattern
as the urine drips down it coats the fins below.

if it just the bottom 2-3" it shouldn't be a problem.

there should be a plate with model numbers on the
outside of the condensing unit..or a plastic tag.
too often the writing fades away. I sort of miss
those engraved tags, even though sometimes you
had to take a pencil & paper & make a rubbing
to read the model number.
compressor only has model number of compressor.

as to cooling maybe set tstat to 76 & see if that helps.
better for unit to run for a long time than to stop & start.
if heat/humidity has built up in the house during the day
then it will take longer to cool at night.
do you set tstat differently on weekends?
how does it keep up then?

I'd play with tstat before calling home warranty co.
it may be that you'll solve the problem yourself.

best of luck.