The United States and Ireland will jointly issue companion stamps Sept. 29 in honor of James Hoban, the Irish-American architect who designed the White House. Both stamps will feature identical designs, except for the designated nation of issue and denomination. The U.S. adhesive will be an 18-cent value, while the Irish stamp will be 18-pence.

The shared design was produced by a pair of artists, one American, the other Irish, who pooled their talents briefly during the summer. The cooperative effort features a profile of Hoban executed by Ron Mercer, the designer of several other Irish issues, and a view of the White House by Walter D. Richards, a U.S. stamp designer perhaps best known for developing the art for the current American Architecture Series. Richard's rendering of the White House shows the presidential mansion as it appeared during Hoban's lifetime, while Mercer's portrait of the immigrant architect features a likeness of him as he appeared at about the time work on the White House was being carried out.

Credit for coming up with the layout for the stamps goes to Bradbury Thompson. Thompson, one of two design coordinators serving on the Postmaster General's Citizens' Stamp Advisory Committee, also arranged the lettering and numerals on both versions.

Appropriately, first day ceremonies for the U.S. stamp will take place at the White House, with First Lady Nancy Reagan hosting dedicatory celebration. Postmaster General William F. Bolger will be among the dignitaries attending the White House ceremony.

Ireland will release its Hoban stamp at a first-day ceremony in Dublin.

The joint issuance appears to be only one of a series of events over the past few years aimed at strengthening Irish-American cultural ties.

Last September Ireland's minister for foreign affairs, Brian Lenehan, seems to have captured the flavor of the joint release when he told an American audience, "The countless men and women who sailed from the 32 counties of Ireland over the last 200 years carried little in the way of material wealth. But they brought with them their intelligence, their courage and sense of pride. With little worldly advantage, but inspired by a positive faith in their ability to triumph over adversity and hardship, they became one of the great stocks of America and helped to build the most powerful democracy the world has ever known."

Hoban in a way typifies those remarks. Born at Desart, County Kilkenny, in 1762, Hoban studied architectural drawing in the school of the Dublin Society during his youth.

As an immigrant to the United States following the American Revolution, Hoban first established himself in Philadelphia and later moved to South Carolina, where he designed that state's Capitol building. He ultimately settled in the District of Columbia.

Arriving in 1792, Hoban found Washington to be a city full of opportunities and he wasted little time in trying to make the most of them. He submitted entries in the design competitions for the U.S. Capitol building and the president's mansion and won the latter.

From 1973 to 1800 Hoban oversaw construction of the executive mansion, a structure he reportedly modeled after Leinster House in Dublin. Originally the home of the duke of Leinster, Leinster House is now the seat of the An Dail, the Irish Parliament.

When British troops set fire to the "President's House" in 1814, Hoban was called upon to supervise its rebuilding, and is said to have been responsible for having the burnt-out shell painted white to hide the burn marks, an action which contributed to the naming of the "White House."

The United States's tribute to Hoban will be a standard-size commemorative stamp printed on a webfed Andreotti press at the Bureau of Engraving and Printing. Printed in six colors -- flesh, orange, brown, green, black tone, and black line -- the stamp will be issued in panes of 50, with one group of single-digit plate numbers appearing in the selvage.

For first-day cover collectors, prepared envelopes, with the 18-cent Hoban stamp already affixed, should be forwarded to: Customer-Affixed Envelopes, Postmaster, Washington, D.C. 20013.

For those who would just as soon have Postal Service personnel affix the Hoban stamp to covers, requests -- and remittance (18 cents per stamp to be affixed) -- should be addressed to: Hoban Stamp, Postmaster, Washington, D.C. 20013.

In either case, orders for U.S. first day treatment must be postmarked by October 14.

Maryland's biggest philatelic exhibition -- BALPEX '81 -- will be held September 5-7 at the Marriott Hunt Valley Inn (located at the Shawan Road exit off Interstate 83 in Cockeysville).

BALPEX is sure to have something for every collector. This year's show will feature a 364-fame exhibit area, 46-dealer bourse and meetings and presentations by a wide variety of national and regional groups.

Among the societies planning to meet at BALPEX are the American Topical Association, the Bureau Issues Association, the International Society for Russian Philately, the American Philatelic Society and the International Society for Guatemalan Philately.

In addition, both the Society of Philatelic Americans and the Society of Israel Philatelists will hold their national conventions during this year's show.

The postal administrations of eight nations -- the USPS; Canada Post; The British Post Office; the Crown agents, which represents 26 countries; Sweden; Portugal; the Faroe Islands; and Australia -- will also be represented.

Also attending BALPEX '81 will be members of the Washington Plate Printers Union. As part of their participation, the plate printers are providing a special show card which honors the first president of the United States under the Articles of Confederation, John Hanson.

Show hours are 10 a.m. to 7 p.m. on Saturday and Sunday, and 10 a.m. to 4 p.m. on Labor Day.

Admission is $2 a day (or $4 for all three days). This price, however, does not apply to senior citizens or children under 12. They pay 50 cents. COINS

One of the question marks surrounding the planned conversion of the cent from its present 95-percent copper and 5-percent zinc alloy to a copper-coated zinc-based coin later this year is whether the new version will corrode to a point where the public will find it unacceptable. Opinions on this vary.

Mint officials feel that under normal circumstances corrosion won't affect the coin's appearance in the least, and they use the findings of experts and the results of sophisticated tests to back their claim.

Some trade groups, however, disagree, and they too back up their position with information from experts and test data.

The Copper and Brass Fabricators Council, a trade organization that has opposed the composition change from the start, is one of those which is far from predicting a tarnish-free future for the proposed cent. In a statement before the House Sub-committee on Consumer Affairs and Coinage last March, council president Robert J. Warfell voiced concern for the new coin's potential for corrosion. With an outer plating of only 2/10,000 of an inch in thickness, breaks in the coating could produce problems, he predicted. "The manufacturing process itself will leave breaks in the coating, and further breaks will develop as the coin wears. Once there is a break in the coating, the zinc core, in contact with the copper plating and the atmosphere, will corrode in precisely the same way that the zinc plates in a storage battery are consumed during the battery's life," he told Congess.

A similar view is held by W. Stuart Lyman, vice president of technical services for the Copper Development Association. According to Lyman, the new coin's composition will pose "a very serious, and probably fatal, obstacle to public acceptance of the new coin."

The Mint doesn't feel that the potential effects of the electro-chemical reaction between the two metals will be a significant problem. To support that stand, the Mint points to tests with copper/corrosion tests, noting that rim wear did develop on coins with a 1/10,000 inch copper-coating, but those with thicker shells weren't affected much at all.

The test procedure used, noted one Mint official, produced far more reliable data than some of those which could be used against the new cent since it duplicates, in an accelerated way, conditions which affect coins in actual circulation.

One thing that those on both sides of the corrosion question do agree on is that, barring a court decision to halt progress on the conversion, the public will be the final judge of the full extent of the problem -- if there is any.