Husband and I started on our first flip in January. I thought I was prepared.  Had done a lot on research on internet.  We spent first couple weeks emptying house of boxes and general trash. First mistake was we started upstairs. The easiest area. We painted all the rooms and cleaned the carpet. Then we started downstairs. The big trouble zone. Previous owner had a dog that used the floors a lot for his bathroom. Dog was luckily contained downstairs.  Floors were wood.  We had tried over weeks that we were working upstairs to pour all kinds of chemicals on them to eliminate the smells.  We soon figured out that the plan to salvage the floors was not working.  We ripped them out. So then we began treating the subfloors. I used about everything that was suggested on the internet: Vinegar, Peroxide, baking soda, bleach, combination of peroxide and baking soda, mouth wash. I spent hours scrubbing and scrubbing. There are 4 main hot zones.  We put down Zinsser Bin Shellac down. Hot spots got 2 to 3 heavy coats. Whole floor got at least one coat. Then after much research and paranoia I also bought 2 gallons of Kilz Original and put that down over whole floor, the trouble areas hit hard again.  We left the baseboards attached when taking out floors. I painted them all over with BIn. I think most got 2 coats. I painted all the walls. Painted the trim with 2 coats of paint. Everything seemed to be smelling good.  We put down roofing paper and put in a 14MM Laminate. We are finally working on the finishing touches. But some of the smell seems to be returning. I have been on my hands and knees so much smelling around the whole perimeter. I have found a few places. I didn't do a good job sealing the trim around doorways.  We ended up just yanking the trim down, I used Kilz on the sheet rock and we put new trim up.  When I go around the perimeter now I am only coming up with one more spot that seems to be with an odor.  I am currently spraying it with Enzymes and then plan to pour as much Kilz in this spot as I can.  This area is closer to front of the house.  But still when you walk in the back door you are hit with an urine odor.  I know that I am extra sensitive to it, but it is there.  Once inside everything seems to smell okay.  It is just when you first open door to walk in.  I have tried to come up with where it is coming from.  I am about at my whits end. I have cried so much about this place.  I don't know how you guys do it. Every time I go home thinking I have it fixed when I come back and walk in and am hit with the slight smell I am devastated.  Realize now that we probably rushed to put the floor down too soon.  But we work 40 hours a week on our jobs and have been working on this place every weekend for 4 months.  I am so ready to be done. I have ordered a small Commercial Ozone Generator.  It should be here today.  We will use it, but I really don't want to use it as a cover up.  I want to make sure I am selling a good clean smelling home.  My knees hurt from crawling around so much trying to find where smells are coming from.  I feel like the floor has to be fine since so much sealant on it.  Smelling the baseboards seems to be fine too.  Any suggestions or advice?