YES! Paste is THICK. That is why another poster gave a great suggestion as to how to get the entire amount out of the plastic jar using compressed air. However, do not do this unles you need the entire amount at one time (like a volume job!)
Here is my Using YES! Paste 101
Be sure you have ample, uninterrupted time to wet mount. You cannot stop and anwer the phone or wait on a customer when in the middle of a mount!
1. You need to gather the proper tools first. To remove the YES! Paste from the jar, we use cut offs of aluminum moulding like #15. The moulding gives you enough leverage to pry the thick paste from the jar. Paint stir sticks crack in half unless you take small glops and time is money!
You will need something to put the paste onto roll it out. We cut/use scraps of glass that are about 12" wide and 15-18" long. We like using glass because it is rigid, flat, easy to clean before use, and it we throw away plenty of pieces this size on a daily basis. After the mounting session, we just throw the glass away. We do not like using a roller tray (or tray liner) because the paste is thick and gets into the recessed areas of the tray and becomes unsuable. It also is a shame to throw away a roller tray (or liner) after use. I do not share Baer's "cover it and resuse it" idea because my experience is that YES! paste crystalizes unless it is very air tite and it makes for bumpy mounts unless it is very fresh. A bumpy mount is a ruined mount, so we do not take chances. You can cover the roller and glue for short duration (like a lunch break).
You will also need a roller cage and roller cover. We use a 7" cage (and sometimes a 3" cage)because a 12" roller is unwieldy and gets very heavy. Your roller cover should have a plastic or phenolic core so it can be reused. Do not skimp on the roller. Cheep rollers shed after repeated use and shedding means a lumpy mount.
You do NOT want a foam roller because you need some "nap" to pick up and spread the paste. Get the shortest nap you can find, often marked as for "smooth walls"
You will also need a hard-rubber roller about 7-10" wide with a CAPTIVE roller that cannot pop out under pressure and have the handle/frame drag across your poster/item being mounted!
You should have a spay bottle filled with preferably warm water, but any water will do. Warm water speeds up the process and is more pleasant to work with.
You need a clean rag (or two).
And, here is a tip worth the price of admission- A roll of adding machine tape (paper).
2. Your substrate should be something that responds well to water based adhesives and PRESSURE! Clay coated fome-cor is a LAST resort because the clay coating swells from the moisture of the YES! Paste and if you are not experienced with a hard roller, the edges can make "lines" in the poster" which are more obvious with shiny, dark posters. It also dents easily and is less forgiving if you use too much water in your mount. 1/8" fome cor works better than 3/16".
Good mounting boards are UPSON, 3xx, x,Jet Mount, Rhino Board, and of course, NATURAL Gator. You CANNOT use YES! Paste on White Gator without sanding the surface and it makes a MESS!!!
I learned how to wet mount using YES! Paste and upson over 35 years ago and when I first opened my own shop I could not afford a mounting press and hand mounted EVERYTHING for a year.
Properly done, wet mounting can be superior to dry/vacuum mounting because the water based adhesives relax the poster and as it shrinks when it dries, creases and defects often "self correct". If you have ever dampened kraft dustcovers and seen how tight they become after drying, you have the same effect (to a lesser extent) with wet mounting.
3. Prepare your mouning board by cutting it at least 1" larger on all sides than the item being mounted. If using GATOR, use a Gator Scraper and DRS roller to clean the surface. Wipe it down with rubbing alcohol and then use the DRS tool again. If you do not have the above, (and you should) the edge of a metal ruller works well as a scraper and a tack cloth will also work.
4. Be sure your glass pallette and the stick you are using to remove the paste is clean. Any grit on the surface will transfer to the paste. If you are using a new jar of paste, you do not have to worry abot being sure the neck of the jar is clean, but if you do not have compulsive "keep it clean" habbits, be sure that all dried and crystalized YES! Paste is wiped clean fron the jar before opening it. (Actually, all WET glue should be removed after a mounting session and the neck wiped clean to prevent crystals from forming.) Crystalized paste will not redissolve easily and will mank for lumpy mounts.
Remove about a cup-full of paste from the jar and transfer it to the glass. It will be thicker than real peanut butter.
If this is the first use of the roller cover, reverse-wind some 1" masking tape around your hand and thoroughly pat down the roller cover to remove any potential lint or extra fuzz. You do not want to transfer anything from the roller to the paste.
Use the roller to smash down the paste and begin to roll it out on the glass. Add one or two squirts of water and continue to roll it back and forth on the glass and it will begin to thin and roll out evenly. Use as little water as necessary to make it roll easily, but not enough to make it a liquid. The consistancy of sour cream is too thin. Add more paste to thicken it up. You want it to be fairly leggy.
I know some framers who put the YES! Paste in a blender and add water to a pourable consistancy for voulume mounting. They pour it our, use a soft rubber roller (like used to apply ink to linoleum blocks) to spread it out. This is a great technique, but is for volume situations, usually using chip board as a substrate and it takes along time to clean up. I would rather buy a liquified dextrin adhesive and use my Potdezin NTZ gluer, but that is for aother post.
Starting with the bottom third of your mounting board, transfer paste to your board with the roller by mnaking a "W" and then fillilng in areas. You are not painting, so don't get carried away to make everything even at this point. GET THE PASTE TO THE BOARD! Continue applying the paste to the board, moving a section at a time. You will need to keep adding paste to you glass, rolling it out and transferring to the mounting board. It will become much faster and easier as you begin to learn the properties of the glue.
Practice on some scrap posters at first and you should also try mounting KRAFT paper as it is uncoated and unforgiving with wrinkles. The liners from Tru-Vue's glass boxes are excellent practice pieces. If you can flawlessly mount one of those sheets, you can mount just about anything.
You need to learn how to balance the water/glue ratio and how wet to apply the glue to give it more "open" time. I can't explain it here asit is a "learned" process achieved by experimentation (and is also based on temperature/himidity of your room). I use wetter paste for larger mounts to give me time to get the glue on the board and roll it out evenly. I used drier paste for smaller mounts.
You will also learn, as Baer states, by the sound of the paste on the board. Once you have enough glue rolled out, there is tacky sound from the nap of the roller sticking to the glue. The sound needs to be the same for all areas of the board. This means the glue is evenly applied and is the proper degree of wetness.
You will also have an even glossy sheen to the glue on the mounting board. Too much glue is better than not enough, but most people put too much on at first because they are afraid of using too little and they think they are painting.
With the proper ratio of glue/water and application, the glue will "sag" or relax a minute or two after application and "flow", so after puting on what you think what may be too little, wait a minute and see how it flows. You have a LONGER open time than you mak think, and if the glue dries too much, it can be reactiviated by a second rolling of a damp roller with a little paste.
5. Unless your poster/item being mounted is small, or you are very experienced, you should have a helper to put down the poster. Start with one end of the poster and "tack it down" with finger pressure on the glue leaving about an inch from the edge. Using the roller, smooth the poster down removing as much air as possible, but do not be concerned with getting the poster flat until the entire poster has been put down.
Working from the middle, work out the air with light pressure on the roller to about 1" from the edges. Take your adding machine tape and roll out pieces larger enough to cover the excess glue on the exposed edges. (Grest tip, yes?) Do not overlap the adding machine tape over the poster, butt it up to the edge. Then finish rolling out the poster all the way to the edges (and past the edge of the poster onto the adding machine tape.)
If you see "chud" under your mount, slowly peel up the poster and remove it. Sometimes it is imposible to see, but you will know it is there and can feel it either on the back of the poster or on the mount board. Then simple reroll down the poster.
If you have a vacuum press, rolling out the air is less critical because if you applied the glue well, there is sufficient slip for the air to evacuate when a vacuum is drawn and all bubbles will go away with a 5 minute cycle.
Any glue that transfers to the hard roller (or your fingers)can/should be removed before transferring glue to the surface of the poster. If the poster is also clay coated stock, a damp cloth will remove paste from the surface without damage. Try to remove the glue when wet, but it can easily be removed after it has dried with warm water. BE VERY CAREFUL with non coated stock because it if is too wet, it will fuzz and pill. Dried paste can be removed with a cotton swab and warm water, just go slowly and do not get the surface too wet.
Weight down the center of the mount if possible, we sometimes put a piece of fome cor about 12" x 12" and the jar of YES! paste on the center as a weight.
Important note. If you are putting the poster in a vacuum press, and are using a cover sheet (as you should) be sure that there is no glue on the surface of the poster, and/or be sure that you use a silicone coated sheet). If you use Kraft paper, you could glue the paper to the surface of the poster and it will tear as you try to remove it. If you do find an area that is sticking, you can dampen the kraft from the top and if given enough time, it will pull off easily and the glue can be wiped down.