male dog marking

Question
We have a 20 month old male lab and a 8 month old shih tzu, female.  My lab marks everything inside and out.  He pees on the kids slide and now we have noticed our patio furniture.  How can we stop this, it is driving me insane.  We watched him yesterday and caught him in the act of marking the furniture and yelled at him, but it did not stop him.  How can we get him to stop marking in the house?  the shih tzu has some potty problems too, which then he goes and marks on that. But he marks other areas that she has not been as well.  
Also, we plan on getting both of them fixed here in the next month or so.  Will this tone down his marking or is it too late because of his age.  
Thanks and I appreciate any help you have to offer.  Do they make zap colars for this?

Answer
This is difficult to deal with.  Neutering should help, but it is not a cure all.  Sooner may have helped, but I am not sure there is good data on that.  

Obedience train both dogs.  Having a good pack structure reduces such problems.  The dogs see all the
people and dogs in the household as a pack with each having their own rank in
the pack and a top dog.  Life is much easier if the 2 legged pack members
outrank the 4 legged ones.  You can learn to play the role of top dog by
reading some books or going to a good obedience class. A good obedience class
or book is about you being top dog, not about rewarding standard commands with
a treat. Start at http://www.dogsbestfriend.com/