Seams, Seals, and
Sealants, PART 2
by
Brian Keys, former Country Coach, LLC
technical resources specialist as written several years ago
Let’s take a look at the types of
caulk or sealant used on your motorhome and how to apply them.
If you have never used
a caulking gun to apply a bead of caulk and you do not wish to learn through
trial and error then you may want to leave this task to a service professional.
Though deceptively easy when watching the process is almost an art form
requiring a blend of finesse and experience. The properties that make caulking
compounds effective at sealing your coach (their tackiness and defiance to
solvents) can also make them difficult to apply without making a mess of your
coach and your clothing. A steady hand and a roll or two of paper towels are
pre-requisites.
Choose your caulk. The colors of caulk
used on the exterior of your coach are usually limited to black, white,
aluminum, or clear. The choice of curing agent is often overlooked and only
becomes critical when applying caulk to surfaces that are chemically sensitive.
A good example is coated or mirrored glass which often requires a “neutral
cure” caulk to avoid discoloration to the coating. A neutral curing agent is
also preferred for interior applications to avoid “fish and chips” odor from
the acetic acid used in many silicones. These two decisions aside, the last
choice you need to make is the most important: the base.
The common bases (in increasing order of effectiveness) are
Latex, Acrylic, Silicone, and Polyurethane. Latex is an inexpensive
caulk that is easy to apply and is water washable. It is easy to clean up,
dries quickly and can be painted which makes it popular for home construction
projects. It is not particularly durable so you will not find it on your coach.
At the opposite end of the scale, Polyurethane
based caulks are the most difficult to apply and are very difficult to
clean up. Citrus based caulks are
the most difficult to apply and are very difficult to clean up. Citrus based
solvents work best if they get on your hands, but some will stain your skin for
weeks until they wear off. They are among the most durable of all caulks and
will cling tenaciously to just about any surface applied to. You’ll find
Polyurethane caulks on the seams of your slide rooms and around the perimeter
of your one piece bonded windshield.
Silicone based caulks are almost as durable as polyurethane
and are relatively easy to apply though clean up remains a chore. These caulks
are very resistant to most solvents. However, they will give in eventually to
mineral spirits or adhesive remover. Warm soapy water should be avoided until all
traces of silicone have been cleaned up-a lesson many of us learned the hard
way. The caulk’s high durability, flexibility and resistance to solvents make
it an excellent choice for interior seams in galley and bathroom areas. It is
also used ion the coach exterior where a visually appealing narrow bead is
required such as along the belt moldings and around the perimeter of framed
windows. Silicone’s main downside is that it cannot be painted. However, clear
silicone can be applied over painted finishes as this is the case with
your coach exterior. There are many
hybrid bases such as Acrylic/latex and Siliconized Acrylic-Latex. However,
these are typically aimed at home construction and are not used on your coach.
Choose your caulk gun from many varieties ranging from the
cheapest that pinches your fingers and won’t release when you want it to, to
expensive air or battery powered versions for those who use their tools
regularly for construction work. If you need an inexpensive but function caulk
gun, choose one that is easy to operate requiring the least hand pressure. A
button found on the plunger of a midline caulk gun will quickly release
pressure from the plunger ensuring caulk stops flowing immediately at end of
your bead.
Preparation,
Application and Tooling.
Preparation – When caulking a joint between two surfaces
that have not been sealed before then preparation in this case is simply
ensuring both surfaces are clean and dry. Wipe them with clean towels and denatured alcohol to remove any traces
of oily residue. It is critical to use denatured alcohol for the final
preparation as alternative solvents will often leave contaminants on the
surface being sealed. Any loose or flaking paint work will have to be removed
or repaired prior to caulking. When replacing aged or weathered caulk, remember
new caulk doesn’t stick to old, so you will have to remove it first and that
removal process requires a suitable solvent together with a lot of elbow
grease. The trick is to remove as much caulk as possible first with a careful
use of a razor blade or other sharp implement, then remove the remainder of the
caulk using a slightly blunt plastic scraper while liberally dousing it with
the solvent. Adhesive remover and mineral spirits will work for silicone based
caulks and to a certain extent, polyurethane based caulks too. Citrus based
solvents are more effective for polyurethane caulks, however, they must be sued
with caution as they are pretty effective at removing paint also. In either
case, ensure the area you are working in is adequately ventilated for the large
amount of solvent required. Once the surfaces to be sealed are completely free
of old caulk then it is time to wipe them down with clean towels and denatured
alcohol to remove trace contaminants from the other solvents.
Application – It is critical before
you begin to ensure the surfaces being sealed fall within the temperature
requirements of the caulk for application. These temperatures are typically
provided in the instructions on the side of the tube. Surface temperatures that
are too cold will affect the caulk’s ability to adhere to the surface while
temperatures that are too hot can cause the curing agent to release too fast
resulting in unsightly bubble formation.
There are as many techniques for caulk application as there
are ‘caulkers’ who claim their way is best. Here are two techniques. Those who are a little short on experience may
prefer to lay masking tape on either side of the joint to be caulked to ensure
a neat straight caulk line and to reduce the cleanup. The downside of tape
lines is it always leaves a small step at the edge of the caulk bead when it is
removed. To minimize this step and to reduce the risk of the bead lifting as
the tape is removed, don’t place tape too close to center of the joint to be
caulked. The recommended distance for a caulk bead is perhaps ¼” to 3/8” from
center of joint.
Those with experience may wish to skip masking tape and
instead use the soap solution technique described under Tooling.
Next load your caulk
gun with a tube of caulk and cut the tip at a 450 angle. The width of the tip
should be approximately three quarters off the width of the finished bead you
are aiming for. Some tubes of caulk will require you to insert a long metal
probe into the tip to puncture a foil seal at its base.
Application begins. Do you push or pull? Most will prefer to pull
the caulking gun away from the beat since it provides good visibility of the
bead as it forms and tends to favor smooth application and consistent
results. Note that the flat surface you made on the tip when you cut it is
positioned so that it forms a triangle with two surfaces being sealed. The bead
itself is a triangle also. It is
critical to ensure caulk bead makes good contact with both the surfaces being
sealed. Some prefer to “push” the caulking gun so the bead is formed in its
wake. Either technique will yield the same end result for most beds. However,
this method is preferred if there is a slight gap between two surfaces being
joined. Whether you push or pull the
trick is to only apply enough caulk to ensure consistent contact with both
surfaces. Too little results in voids in the bead; too much makes tooling
and final cleanup a chore.
Tooling – Once the caulk has been applied, all that
remains is to smooth out the bumps and feather the edges for that professional
finish your grandmother would be proud of. This must be done immediately after
the caulk is applied (before it skins over) and you only have one chance to get
it right because subsequent tooling attempts will quickly turn an acceptable job
into an unsightly one.
If you applied masking
tape on either side of the joint area then spray a finger of your choice with a
mild soap solution and drag it smoothly back over the bead of caulk with enough
pressure to ensure it stays in contact with both surfaces being sealed.
You’ll need to stop
every now and then on long beads to clean your finger of excess caulk and
reapply some soap solution. Keep an abundant supply of shop cloths or paper
towels ready. The amount of mess you make during tooling is often inversely
proportional to the amount of towels allotted for the job.
Once the bead has
skinned over, meaning the surface is no longer tacky to a light, it is time to
remove the tape. Peel the tape off slowly and keep it from coming into contact
with your clothes or the coach since it will probably still have wet sealant on
it.
If you decided to
forego masking tape in favor of the soap solution technique then simply spray
the entire length of the bead with a liberal amount of the soap solution. Now
spray your finger with the same solution; follow instructions above for masking
tape technique. The soap solution will prevent the excess caulk from adhering
to surfaces being sealed and will produce clean feathered almost invisible
edges.
You can wear latex or
nylon gloves, however, the gloves should be a tight fit to avoid balling up at
the end of your finger and preventing a smooth bead. Some prefer to use a shop
cloth or thin dish sponge tightly wrapped around the end of finger but may
cause grooves in the bead, or worse still, remove too much material during tooling.
Latex or nylon gloves are recommended for polyurethane caulks.
Roof Caulk – While these instructions are typical for most
caulk used on the roof of your coach requires a slightly different technique.
This caulk is a rather special self leveling acrylic caulk that has proven to
be very tolerant of the harsh environmental extremes the roof of your coach is
subjected to. Preparation of the area to be sealed and removal of old caulk is
the same as above. However, caution is needed in the application to ensure you
don’t end up with a bead that is much wider than you intended. A good rule of
thumb is to lay a bead that is 1/3 to ½ of the finished width you are aiming
for. Within minutes, the bed will widen as the self leveling caulk flows.
This caulk requires
particular attention to the temperature of the surfaces it is being applied
because it can fail to adhere completely if the surface is too cold and will
produce hundreds of pin holes as the curing agent releases if the surface is
too hot. Pay heed to the instructions on the caulk’s container for ideal
application temperatures.
Practice Makes Perfect! As you
can see, caulking is not as easy it might seem and is certainly labor
intensive. It only takes a little practice to obtain good results. Fortunately
the caulk or sealant itself is relatively inexpensive so you can afford to
waste some while making a few practice runs. Apply some packing tape to two
long pieces of wood and clamp them together into an “L” shape for a good
surface to practice your beads on. If you don’t get it right, simply peel the
tape off together with the sealant, reapply the tape and try again. Different
soap solutions or ‘surfactants’ will yield different results and studies have
shown that non-iconic surfactants work best followed closely by anionic
surfactants. Cationic surfactants are not recommended for this task. A week
solution of 409 or Lysol seems to work just fine.
Common
Caulks and Applications
Dow Corning 733 – 100% Silicone.
Used on belt moldings for skirts, roof, front/rear caps. Used around framed
windows. Cannot be painted.
Dow Corning 737 – 100% Silicone,
neutral cure. Used to adhere trim and mirrored glass on coach interior.
SikaFlex 221 – Polyurethane. Used to seal
slide walls and other areas where cosmetic appearance is not so critical. Can
be painted.
SikaFlex 252 – Polyurethane. Used as an
adhesive for exterior trim and stainless steel panels.
SikaFlex 295UV – Polyurethane.
Used around the perimeter of one piece windshields and frameless windows. UV
Stabilized.
Schnee Morehead SM5504 – Self leveling
Acrylic. Used for most roof seals.
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