I have two 8 month old male whippets. They are brothers from a litter i had last year. Even with my patio door wide open and regular walks etc they are 'marking' around my home, which is very frustrating. One of the boys is very docile while the other is quite hyper but between them it would seem they want to be top dog. When i take them out for walks they nearly always mark on top of where the other has just been. I have three bitches (2 spay & 1 not) i have never had this problem with the girls as they are so clean.
Can anyone advise me on how to stop this please. Do sprays work? I'm at my wits end with them. It is very rare that i catch them red handed doing the deed and when i have done i send them straight out into the garden.
Any advice would be greatly appreciated, thank you. 

Complete supervising and a sound telling off when caught in the act of about to do it.

So keep them crated confined at all times you cannot watch every move.

From their point of view they are doing nothing wrong, can have no idea it is unacceptable, only what comes naturally.  It isn't a house-training lapse, but a behaviour we find unacceptable, like chewing the furniture is not on, but chewing an appropriate toy etc is..

Thank you for replying. I think i must have psychic dogs LOL! Since i posted this query neither of the boys have marked anywhere in the house. I don't have crates so kept them confined to one room with back door open, rain or shine. Didn't reply sooner as i didn't want to put the mockers on it! But...... so far.......... a week after your reply all is looking good! Thank you!

Think this may be the teenage thing, adult dogs accept that things are yours and therefore they have no right to mark, agree with Brainless if it is your chair or tree then let them know in no uncertain terms it is yours and they leave it alone. It is one of those behaviours that you need to catch in the act tell them no but reward when and if they do as you say and return to you. 

As the testosterone increases the urge to let everyone know they are available increases as well but it will go once they are adult but the habit of peeing on your things or even you will not if it is not corrected now.

I have a dog that does this after he's had a bath--he has to be watched like a hawk and even though I take him out before I get him in the tub he always manages to pee on something. He's the only one who does this.

This one has been a problem for us too! We have a 20 month old Jack Russell bitch and a 16 month old Jack Russell dog (she's hyper and he's docile) who occasionally marked around the house. I found lots of praise worked wonders. Each time Pippa went for a wee we praised her so much that Jake followed suit (as he doesn't like being left out) - then praised him to the hilt. He now runs in tail up, as if to say look at me - what a clever boy!!! We also calmly reprimanded him each time (he hates being told off). Apparently lifting their hind legs (if you can catch them in the act) whilst marking stops them mid-flow and then you can take your dog outside. Yes, girls are much cleaner :)