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cc Choosing the right compound for drywall joints by Earl Maynard. Manager. U.S.G. Drywall Finishes Research Since fhe advent of drywall construction in the early 1920's, manufacturers ond drywall contractors hove wrestled with the problem of how to conceal and reinforce the joint. Over the years many methods hove been tried. Some were absolute failures and others only relatively successful. Some products olone do an adequate job while others in combination do a superior job. Unfortunately no one product solves any one problem completely. The net result has been the development of a family of nine dry powder and three reody-mixed joint compounds. Each is designed to do a specific job under a speci- fi edition. Thus, the careful se lection of the proper joint treatment materials by the drywall contractor is important to finished job results. This report out I ines the basic types of joint compounds, their composi tion ond recommended usage. It is hoped that through better under standing ol the products, mony of your joint treatment problems will be solved. Bear in mind thot joint JJEl.UJ, -Ur-jiLl ji != i itr. treatment compounds for drywall Tf-jrr construe!ion have two primary func tions: I) joint reinforcement and *"i,i 2) joint conceclrnent. History of Joint treatment Ecrly joint treatment of SHEETROCK* Gypsum Panels consisted of plaster of-poris forcoJ into the joints. Severe creel ing invar- x A. *1 , icbly occurred in nearly all joints. This was followed by an ottempt lo ,__ reinforce the joint with a narrow Cr` A 1---- cloth tape embedded in the plaster. ''V'V.t The tope helped, but v,-us for from ""y/// liy I satisfactory. Ir became clear that a strong tope and o strong adhesive compound werp needed to replace the brittle plaster. One answer to the orobb-m was a zinc alloy tape called Metal "A" introduced by U.S.G. It was applied with a strong, "high alkaline" joint compound. ' The tape had large perforations and plenty of strength, hut was expensive, hard to handle ond an outline of the tnpe "pholoqtophej" thiough finish.-d walls. Continued research eventual Introduced a higfi- strength paper tape. It v.os eosy to handle and inexpensive. Several modifications have been n>eje Sin:e hut paper loot.' iv still used universe1 i y for joint reitd sreernent. from the ISM.O's to the present, most dryw.ill construc tion improvements iuvo been made in the t ompPUivis and .!;!. fyrrnotion o! the panels themselves. Basic Types of Joint Compounds Functionally there are three basic types of joint compounds in use today: taping (embedding) compounds, topping (finishing) compounds and all purpose (or multi-purpose) compounds. Each is designed for a specific purpose. /Taping Compounds _____1 A ^ fj* IT*** A t\ \ 1A 25 9 ! free"**: Tap ing compounds, sometimes called embedding compounds, ore used to embed the joint tape. They have the greatest adhesive strength of oil the compounds. Since their function is to rein force the joint, they are fhe best products for this use. They ore also best for the first coat on metol corner bead ond trim. The strong bond also gives superior corner reinforce ment. Because of greater adhesive strength they con be thinned slightly for use with mechanical taping tools. Taping compounds are also frequently used for first coat application on fastener heads. They may be used for finishing coats but they're border to sand. Topping Compounds r L_ _.i Topping or finishing compounds are used for the fill uiid final roots over the embedding compound to fully concecl the joint area, fas tener heads, corner bead and trim. To finish for decorating, two coats of conventional topping compound are required. Deep fills ond poor drying conditions sometimes make additional coats necessary. SZOdO * ^ .p I u- ! . * V f \ junJ'- i ivivr: u |! 'tit . '.) f il ** orvi ` v : " t'J 'c?nd. ISvy :ro <( ?n*-j K i..tve itirf l.-.'st *.. '-. v I - i 11 ty in louls, 'm.-.'N i:<.".t l.'itur*' rind dir I :!. .--;l*. fhey nri.` I lie Lust products for linisiui'i join;*, since th^y'ir spec i li cal I y du'.igncd for t Iris , Hire: ost.'. All-cutcc:'.1 Compounds All-purpose* compounds offer (lie convenience of a reinforcing, fin ishing or a texturing compound in one package. However, all-purpose compounds ere a compromise be tween the Superior bonding quality of a taping compound end the supe rior finishing properties of topping compounds. What's in a Joint Compound? Casein Compounds Joint compounds, in essence, are wuter-based adhesives. Until the 1950's all joint compounds were casein-based, dr/ powder products.: Tadcy cc-seirrccmpourds-ore-Mill used-fn mcnyAirecs, ^ ^ including PERF'vA'-TA'Pt'GoVnpO'jnds, but they one rapidly be ivd replaced by either ready-mixed compounds, non-casein compounds or herdeningtype compounds. Casein is the glue that binds the inert fillers and tape to the gypsum wallboard. Casein is a natural product separated from skim milk; once it is separated and dried for use in joint compounds, it will not dissolve in water. To serve its pur pose, however, it must dissolve when the joint compound is stirred into water. Since casein will dissolve in alkaline solutions, joint compound formulae incorporate alkaline re acting salts in small amounts. When the compound is mixed with water, the salts dissolve and produce the alkalinity to dissolve the casein. Eorly casein-based joint compounds were commonly called "high-alkaline" compounds. A heavy concentration of alkoli salts in the compound dissolved the casein quickly, even in cold water. However, the alkali reacted with certain types of paints, referred to as "burn-through." Burn-through, and other problems related to high alkalinity, led to development of "low-alkaline," ca sein-based joint compounds. Most manufacturers simply reduced the amount of alkali. U.S.G. changed the-entire alkaline solvent system to a mild ammonia type which evaporates during drying. There is no alkali left to cause decorating problems. 'n Lhiir") ng i lv . nV- .. m t ir-n i* I n c _*rn::.jtjnd irvi IV* f'*rT.iil<i lo 'inn'ti' i oilrn present', new piobien ; t'o lew -clkol liv *i :nd*., wlii I tIvy don't but n t!' cu h. iv o two trill' r - cut discnjvnntu <5 . Pecowe of iow alkalinity, the comA: n binder tukes lonmv to dissolve, especially in cold v/nti'i . That's why most monu- focturers specify a 3Q-rnin. soaking period. Another problem: most ca sein compounds will thin cut Dr water-down when they stand for Icing periods after mixing. r These disadvantages led to development of non--: casiifn compounds. Adhesive binders in these powder compounds require no alkaline sol vent. The binders themselves ore water soluble. Non-casein com pounds are uscble os soon as they ere adequately mi.\ed--and usually stable against thinning out. The non-casein compounds have the ad vantages of both the high- and low- alkaline casein compounds and none of their disadvantages. These compounds are rcpidly replacing the casein types. Ready-mixed Compounds J------ The finest ond certainly the fas test growing joint compound in the industry is the ready-mixed com pound. The manufacturer has teken the responsibi I i ty of mixing the com pound too smooth, lump-free consis tency. There are many obvious ad vantages in ready-mixed--no need to find and carry water, no mixing, no waiting. Agplication of ready- mixed, compared to casein powder compounds, is "slicker." Ready-mixed has superior crack resistance and ease of handling in tools. It is truly an all purpose type product, but superior to casein powder all-purpose compounds. Ready-mix usually has better adhesion without excessive hardness for sanding. Vinyl-type binders give ready-mix many of these properties. /, _ .... ................. - - . Drying- and Hardening-type Compounds There are two basic forms of joint compounds--the drying type ond the hardening type. The dryingtype joint compound must dry by evaporation of water to become hard and develop full adhesion. Drying time depends almost entirely on the temperature and humid ity. Little can be done by manufacturers to speed 8 ZOdD --orxax. tr s'yiw down the drying rote. All pounds will dry `fast on a warm, dry dav and very sk .y on a cold, damp day (see"Drying Standards" on page 4). Drying compounds shrink when they dry. Some formulations shrink more than others--but they a]_| shrink. The shrinkage problem led to development of hardening-type compounds . . . minimal shrinkage compounds. After compounds chem ically harden they do not shrink -- thus are extremely valuable in deep fills such as on metal corner bead and in openings around pipes, etc. The primary value of hardening-type compounds. 35 ) such os DURABOND* Joint Compound--Taping, and DURABOND 90 Joint Compound, is the shorter time required to finish joints. As soon os the DURABOND Taping lies hardened--usually less than 4 hours--topping coat moy be applied (DURABOND90 hardens in approximctely 90 min.). This brief time period is especially valuable in cold, damp weather when it is often necessary to wait several days before drying-type compounds are ready for another coot. With proper job sciiedul ing ond adequate man power, the three-coat joint treatment on SHEETROCK SW Panels can be completed in a single day. This is sometimes considered impractical or unnecessary. But frequently, pre-filling, taping end fill coats may be applied the first day and final topping coat the second. USG " Joint Compound--All Purpose, U5G Joint Compound--Topping, USG Ready-Mixed Joint Com pound-All Purpose or USG Ready- ^7,,^ . Mixed Joint Compound--Topping are recommended for the final coat over DURABOND. These products are '''5 easier to sand ond touch up forfinul decoration. They clso give a more even suction across the joint. Thus, V t- I hardenir.g-lype compounds vri11 not reduce the number of coats, but they give considerably more assurance that no more than three coats are -i L. requiied, evert in slow-drying weother. DUivABOND XL Joint Compound is cn exterior joint treatment formulation in powder form far pre-filling, embedding tope and linishirn joints of USG Exterior Gy O',urn Ceilir.n Coard--clsj far tilling holes in board '.urinous, t I induing ond filling metal accessories and fusti usi heads. Unlit.other DURABOND compounds, DURABOND XL is formu lated for all three joint- treatment coot* --embed ding, filling and finishing, if necessary. However, each coat must be applied witliK 'fcssional smoothness in order to ovoid sanding ana\..iishing. DURABOND XL Joint Compound hardens in from 30 to 60 min. The advantages of a hardening-type product are most obvious in slow-drying weather. But what happens when DURABOND Joint Compound is used on a hot, dry doy, and dries before it hardens? In such cases a normal amount of shrinkage occurs--about equal to that of ready-mix or casein powder compounds. In other words, DURABOND will perform about the same as a conventional drying-type joint compound. DURABOND Joint Compound--Taping, DURABOND 90 and DURABOND XL Compounds are U.S.G.'s hardening-type prod ucts that a skilled drywall mechanic can use effectively. They provide the opportunity to gain speed and quality where his job calls for it. At the same time, he can accommodate a wider range of job conditions. Compatibility of Joint Compounds As described cbove, each joint compound i. de signed to provide a distinct job result. Each product within each line is designed to ac complish a specific job result. The chemistry of each line frequently |f differs and is often incompatible if* with cnother line. The incompati- bil ity arises when products from dis- ' similar lines become intermixed-- either in the mixing container or j\ on the wall. Complete loss of bond / \\ or impossible application properties can be evidence of contcmination. Cosein compounds generally are incompatible with non-casc-in compounds and with hardening-type joint compounds. While cosein formulas are fairly tolerant of ready-mixed formulations, the two lints were never intended to, nor should they be mixed together. The aryv-oll contractor should be aware of the need for job cleanliness .. .cleanliness of mixing con- 1111 LI 1 \ tamers, tools water, et cetera. With the introduction and use of new product lines, job cleanliness becomes increasingly important to the Joint Treatments for the Future Every manufacturer is looking hard for new answers to gypsum drywnll joint tieatmenf problems. This is true at U.S.G.'s Research Center where the most recent developments--SHEETROCK SW and its companion, DURABOND 90, were developed and released for manufacture. New joint compounds and techniques are now being developed for the manufactured liomc industry w-here panel joints must be firmhed in a matter of hours instead of days. Perhaps the future will see a practical and effective one-coat joint treatment system. o X o ra 00