Stains Bleeding Through Wallpaper

This is the current wallpaper, and I’m to put a new paper in this powder room.  For various reasons, this paper cannot be stripped off the wall , so I’m prepping the walls to be able to hang the new paper on top of it .  Normally, my favorite Roman Pro 977 Ultra Prime will seal this effectively enough.  But – hard to see in this photo – but this existing paper looks dirty to me.  Some of that is due to the mottled look of the pattern / colorway .  But I’m seeing tan discoloration , and I think it’s coming from under the wallpaper . 
This home was built in the early ‘90’s , and in a neighborhood where , through stripping off many old wallpapers in order to prep for new installation , I’ve noticed that someone working in that area and era used what we call clay-based paste .  This paste is made using actual clay – i.e. dirt – and has a reddish color .  I  believe that this red clay paste is able to work its way through wallpaper, and creep onto the surface, causing a subtle “dirty” cast to appear on the wallpaper. 
Once you strip the old wallpaper off the wall, the wall can then be washed , and hopefully you’ll get all the clay paste off .  But usually, no matter how much you scrub, residue will remain.  I fear that that residue will work its way through the new wallpaper, as well. 
Additionally, the most severe stains are in the vicinity of the toilet – housekeeper splashing cleaning agents?  or little boys with bad aim ??  
Yes, most homeowners will never notice this, especially since it can take years to show up.  And the paper I’ll be hanging is black , so it’s highly unlikely that any discernable stains will become visible .  But I get a little anal about wanting jobs to look perfect. 
 

So I like to use a stain blocker , which will ensure that no discoloring elements will work their way through .  My two favorites are KILZ Original , oil-based primer .  KILZ is wonderful, unparalleled stuff.  But it does have strong fumes , so you have to ventilate the room thoroughly and also wear a chemical respirator while applying it, to avoid becoming “high.”  i.e. “sick”   
My other option is BIN , which is shellac – based  .  This is also an excellent stain blocker , and has a strong smell , but nothing that will kill your brain cells .  Since my current clients have little kids, I opted for the BIN . 
Unless you want to go through a LOT of paint thinner and mess , you can’t easily wash oil-based or shellac-based liquids out of your roller cover or brush es .  So I plan to use disposable rollers and brushes.  This doesn’t mean cheap rollers and brushes – it just means that I factor in that these will be used once and then thrown away. 
Speaking of factoring in costs – the gallon of BIN was  $70 (and that was a reduced price due to dented can).  The KILZ was $40 , but I would have had to purchase a new respirator and cartridges , which would have added an additional $40 – some of the respirators went up to $70.  March 2023 prices . All this needs to be considered when figuring up the price for doing a wallpaper job.   I’ll also note that these products can be difficult to source – my local hardware store, which typically has stock even when larger outfits are out , had only one can of KILZ and one scratch-and-dent can of BIN .  Whichever I would have chosen, I sure got lucky. 
Note that these stain blockers are formulated to block lots of nasty stuff – water stains , blood , rust , wood sap / knot holes , ink , tar , grease , crayon , lipstick , cooking stains , marker , Sharpies , etc. 

Back to BIN….  It’s very watery and runny and drippy .  So you really have to cover the floors and baseboards , as well as countertops , toilets, etc.  Microfiber roller covers help a lot to minimize splatters .  Roll slowly and in an upward motion . 
Oh – additional plus … both BIN and KILZ dry quickly, so you can apply it and then move to your next step in less than an hour. 

Besides the shadow-y paste residue , this room has other issues .  Here, someone used ink pen to mark placement of the hooks for the powder room mirror .  Eeek!  Ink will bleed through paint and wallpaper even faster than clay paste residue will.  Here you can see one coat of the BIN applied – but the ink seems to have  been “resurrected” and is actually enlarging and spreading, and bleeding through the stain blocker . 

Here I’ve spot-primed with a heavier coat of BIN , and that appears to be holding the nasty ink at bay . 
Note that wallpaper paste will not adhere to oil-based products like KILZ , and probably not to shellac , either, because it dries kind of glossy.  So whichever of these primers you choose to use, you will need to top-coat with a primer formulated for use under wallpaper – and that will adhere to the stain blocker.  Tomorrow, I’m going to follow up with Roman Pro 977 , as mentioned above .